Do i have hyperthyroidism?

Last august i weighed myself and i was exactly 100 lbs, then i went to the doctor 1-2 months later and i was 96 lbs. i weighed myself again in late january and i was 94 lbs, then i weighed myself a week later and i was 92, a week later 90, and now 89.

i've always been really skinny(right now i'm 15, 5'4'') but I eat as much as anyone else does & i never really exercise.

should i get tested? Does medication for hyperthyroidism make you gain weight? i have a lot of the symptoms listed here (for hyperthyroidism): http://thyroid.about.com/cs/basics_starthere/a/symptoms.htm

i got tested for diabetes 2 yrs ago and i don't have it, but i didn't start losing weight until recently.