Sudden weight loss at around 13?

I'm 14 now and about a year back I had weighed around 120 pounds or so and suddenly lost all of the 20 pounds within about a  month...then stayed a consistent 99 or 100 pounds ever since. I eat pretty bad... (mostly fast food) and don't get as much exercise as I should...and I'm usually always tired and sore. also somewhat dizzy when I first stand up at times. My mom says it's because I don't eat right or exercise enough which I am currently working on...but I am still curious as to what may have caused the weight loss and the other symptoms I currently have? I wasn't doing anything different such as dieting or heavy exercise. Is there anything I can do to start feeling better and more normal? Thanks for the help.

[Hypochondria563]

Usually at that age is when Human Growth hormones are at there peak. HGH stimulates skeletal mucle and bone growth and well as using fat a a primary fuel source, but you need to eat protein in order to build msucle, calcium and phosphorus, B12, folic acid, to build bones. So if your night eating actual "food" food your body has a hard recognizing it as fuel or to activate hormones. Wrong kind or no fat depresses the endocrine system, wrong kind of carbs can make you resistant, diabetes , etc, and you won't grow without protein(or make new red blood cells so you brain won't get oxygen, no muscle tisse, no amino acid hormones, etc) try eating healthy foods and you'll notice the difference in your energy level and development.

[ducky406]

Hypo has some good suggestions but I would suggest seeing a doctor to just to rule out diabetes type 1 and thyroid issues too. Your weight is being constant, and your symptoms could easily come from only eating "junk" foods. However, both are serious conditions that need to be ruled out, especially if you're underweight.

[annie1987387]

Maybe you were stressed or something. You will have to try eating healthily and excercising more but with exercise you have to do it slowly and gradually at your own pace, don't do it to fast though.

[ember214]

Thanks guys :)