Please Advise!

So, I am 20 years old.. normally, I have very irregular cycles (I swam semi-professionally up until I was 18).. I tend to skip 2-3 months between periods. My last "normal" period was late in July of 2012, I had protected sex (and I made him check the condom for rips or leaks after..nothing was wrong) with my steady partner of a year after that.. since then, I have gained about 5 pounds due to poor diet and lack of exercise.. I skipped periods August and September.. which I thought was normal... then skipped October as well.. still wasn't worried.. I broke up with my boyfriend in between that time and was very stressed with school, a new job and family problems. I started thinking that I may be pregnant, since I am not on any kind of birth control and that caused more stress.. however,  Early December I had bleeding. and it was so light that I barely noticed..It lasted for like 3 or 4 days.. it was super light and I had no cramps, soreness or anything! I would think that I am pregnant, but the thing is I have had not had any kind of sexual intercourse since that last time..
With this "period", i thought, okay maybe i'll get back to some normalcy.. but like three weeks later, today, January 1st 2013.. I noticed I was bleeding... again... same thing, very light, barely noticeable.
Can anybody advise on what may be going on? I have made an appointment with my doctor, but I want to know other people's thoughts before hand.. Am I pregnant? Could I be pregnant? My ex-boyfriend and I have gotten tested for STDs and we were in a monogamous relationship (also, we were each others' firsts)..so it can't be that? Infection? I am extremely worried that it might be pregnancy.. but, without any of the symptoms (no nausea, no breast tenderness, pain, vomiting, food cravings... sexual intercourse, even) it seems highly unlikely.. but what else could it be?

Thanks everyone for the help in advance.

[Dr. Kokil Mathur]

Hi!
Basically you have secondary amenorrhea or absence of periods/menstruation due to some thing which persists for three months or more. To understand what’s going on it is important to first take pregnancy test and see. If it is negative, you will need further investigations.
Stress and anxiety is a major factor. Thyroid problems and abnormal levels of FSH, LH, prolactin, androgen hormones or male hormones and estrogen hormones too can be the cause. MRI of brain may be required to rule out pituitary tumors secreting prolactin. Another major contributor is polycystic ovarian syndrome (PCOS). This can be diagnosed by a CT scan or abdominal ultrasound and tests for insulin resistance. Smoking is also often associated with secondary amenorrhea and can be worse in presence of stress. Sometimes tumors of the ovaries and brain may also cause absence of menstruation. Severe anemia and malnutrition, excess weight gain or weight loss, hypothyroidism too can contribute. If you have been on pills or depot shots then this can be post pill amenorrhea (ruled out in your case).
Since a confirmed diagnosis cannot be given on net please consult your gynecologist regarding this. Hope this helps. Take care!

The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.

[Yessitscece]

thank you! Since my post I have taken one HPT and it came out negative.. I will test again after 3-4 days to make sure.

I have also started to have this dull/tugging ache in my lower left abdominal and a dull back ache...I still am seeing small streaks of pinkish or bright red blood when I wipe and now in addition to that I seem to be passing small amounts of tissue in my urine.

Any advice of what this could mean?