Well, I had anorexia for quite awhile so I can probably answer any questions you have.. The questions you asked in your post are kind of basic, so I can't give very detailed answers but... Yes you can have stomach pain along with many other symptoms. Anorexia and Anorexia Nervosa are actually two different things.. Anorexia is simply not eating, it doesn't necessarily mean you don't WANT to eat, just a lack of eating for whatever reason. Anorexia Nervosa (what people are usually talking about when they use the term Anorexia) is when you don't eat because you think you are fat, or you have a strong fear of gaining weight. Though technically, part of the clinical diagnosis of Anorexia Nervosa is losing 15 percent or more of what your normal body weight should be, you do not need to be super thin in order to have an eating disorder. Most of what an eating disorder, such as Anorexia, consists of is the emotional/mental aspect.. being scared of gaining weight, thinking you're fat when you really are healthy, starving yourself or severely limiting food quantity or types of foods, etc. You can be of normal weight and still have an eating disorder.
If you have specific questions, I'd be happy to answer...
To have anorexia nervosa you must have a BMI under 17.5, you must have not have a period for at least three months, have an intense fear of becoming fat, and use unhealthy methods to lose weight.
Anorexia nervosa also has two subtypes, type one is restricting subtype, where the sufferer will restrict their caloric intake in order to lose weight. They do not purge and rarely binge. They may or may not over-exercise also.
Type II is the puring subtype. Sufferers with Anorexia Nervosa Type II not only starve themselves, but they also purge regularly. (Purging does not only include vomiting but also using diuretics, laxatives and so on.) They may or may not also binge eat on a regular basis before they purge.
Someone with a BMI under 18.5 is classified as underweight, and to have anorexia nervosa (either subtype) you must have a BMI under 17.5, so yes, you must be quite thin to have anorexia nervosa, however to have other eating disorders, like Bulimia Nervosa (either of the two subtypes of bulimia) or ED-NOS you don't have to be thin at all. Actually, most people with bulimia or EDNOS are of normal weights or overweight.
Eating disorders are most commonly caused by rape and/or abuse, or other traumatic events. A common myth that people often spread around is that eating disorders (like anorexia) are caused by the media, or looking at thin models and celebrities. (This myth makes me very angry, as I have suffered from anorexia nervosa for over 25 years and am terminally ill because of it. I have never been interested in celebrities or models, I've not watched a television in decades and never even owned one in my life. I don't read magazines and I don't listen to the radio.)
Another common myth about anorexia nervosa is that those who suffer from it lose weight to look sexy, or beautiful.
Many people with anorexia nervosa, due to trauma in their pasts, want to lose weight for the complete opposite reason, to look sexually unattractive.
In my own case, because of a lifetime of being raped and abused, I wanted to become emaciated so that men would find me unattractive and would be sexually repulsed by me, so that they would leave me alone. I hope that by losing weight the abuse might stop.
Anorexia nervosa, as well as other eating disorders, cause EXTREME damage to all of your major organs. Eating disorders leave nothing untouched, when it comes to the severe damage they can do.
I will list off some of the COMMON complications of eating disorders for you.
Psychiatric complications from anorexia, bulimia and ED-NOS:
(Most of these can occur before or after the onset of the eating disorder.)
Depression; social anxiety/phobia; personality disorders; abuse (ED sufferers are at a higher risk for all types of abuse, including physical, emotional and sexual, as well, rape and abuse are the number one cause of eating disorders); suicidal thoughts and actions; low self-esteem; self harm; isolation; aggression; paranoia; psychosis; body dysmorphic disorder; self-hatred; difficultly in relationships; drug abuse and alcoholism; obsessive compulsive disorder; sexual dysfunction; need for psychiatric observation in a locked ward; fear of the opposite sex; lack of motivation to find work, go to school, etc.; anxiety; panic attacks; "forget" how to eat normally; obsessions; easily addicted; etc.
Medical complications from anorexia, bulimia, and ED-NOS:
Fatigue; dizziness; hair loss; motor skill impairment; lanugo (downy hair that grows all over the body after extreme fat loss, as a last ditch effort to keep warm); insomnia; sleep cycle impairment; tooth decay; tooth loss; increased susceptibility to infection; liver failure; kidney failure (extremely common, and often sets in early on during starvation), heart failure; heart attack; anal prolapse; muscle deterioration; ulcers; decreased or absent bowel function; loss of menstruation leading to: ovarian cysts, increased risk of ovarian or cervical cancer, hormonal imbalances, infertility, etc.; lack of calcium, leading to decreased bone density and eventually osteoporosis -- symptoms of which are pain, shrinkage of the spin and limbs, regular fractures in the bones; constantly feel cold; confusion; slowed metabolism; nail breakage or loss; inability to walk; bruising; difficultly clotting blood; difficulty healing from minimal injuries (cuts, scrapes, etc.); incontinence (inability to control one's bowels, leading to 'accidents' in public places); cardiac arrhythmias (abnormal heartbeat); diarrhea; exhaustion; impaired ability to regulate body temperature and sweat production, increasing the risks of hypothermia and over-heating; inability to control urination; bed sores; arthritis; various diseases of the colon, many of which are fatal and necessitate the removal of most of the colon; gallstones; kidney stones; frequent nausea; uncontrolled projectile vomiting after meals (not self-induced purging, stomach will no longer accept food); hepatic problems (liver); memory loss; perforation of the stomach due to purging; tearing your esophagus (throat); choking or inhaling vomit during purging; coma; brain damage due to coma, electrolyte imbalance, etc.; dry skin; slow heart rate; diabetes; irritable bowel syndrome; stunted growth; low blood sugar (causes confusion, dizziness, aggression, fainting or collapsing, seizures, coma and death); low iron/anemia; impotence in males; edema (swelling); increased risk of various cancers; vision loss/disturbances; fainting; collapsing; injury due to fainting/collapsing; hearing loss; circulation problems, and risk of losing limbs as a result (gangrene); acid reflux; indigestion; pulmonary (lungs) symptoms; HIGH risk of kidney and urinary track infections; blockage in the bowels; dehydration; over-hydration (water poisoning); high cholesterol; loss of sex drive; low blood pressure (risk of multi-organ failure, fainting, etc.); spinal shrinkage (hunchback); internal bleeding; digestive track failure; constipation; nerve deterioration; shrinkage of the brain; heart shrinkage; overdose on "regular" adult dose for medication, due to inability to metabolize drugs; and, of course....
Many of the consequences of eating disorders I have personally dealt with and still deal with. (I have never been able to recover from my eating disorder. Currently I am living out my final days in hospice, and am dying from multiple organ failure due to anorexia nervosa, type II.)
If you have anything else you wish to know, I have been through medical school (I studied psychiatry) and as I said in the beginning of my comment, I have suffered from this illness for over 25 years. I also speak out and educate people online about eating disorders through various websites I own and on Facebook, so I should be able to answer any questions you have.