Why aren't I getting skinny!?

I am so frustrated. Since the beginning of June, I decided to work a lot on my diet. I eat under 2000 calories (mostly low carb food, a little protein here and there and the occasional cookie), I drink TONS of water (at least 2 quarts a day), I work out 4-6 times a week for at least an hour, and yet I still manage to gain weight! I eat small portions and once in a while I'll eat a Lean Cuisine. I get at least 7.5 hours of sleep every night, and on the weekends I get about 4 hours more than that.

My workout regime consists of:
Anywhere from 15-30 minutes on a stationary bike with varying resistance,
10-15 minutes on the elliptical with about 50% resistance
10 minutes of jogging 8 min/mile on the treadmill
30+ minutes on various weight machines doing LIGHT weights or crunch machines, medicine ball crunches, etc. (usually 12-25 lbs depending on the machine)

During the school year I play field hockey and run sprints on the track team... I've always felt pretty disproportionate compared to other girls because I'm short...

What am I doing wrong? Seriously I'm considering a detox diet or something extreme because NOTHING is working and I'm pretty fed up. I know the consequences so please refrain from telling me that it's bad. Thanks.
gigidy- You're funny, ignorance and stupidity on this site never ceases to amaze me.

alex- Yeah I've always been muscular since I'm a sprinter, BMI is never accurate for me.
Okay I don't eat "fatty"

By eating under 2000 calories I meant an average 1200 a day. 2000 was my limit throughout track season but in the off season its a lot less
Marissa I'm 5'1, 120 lbs

I know I'm not fat, my doctor and fitness trainer say I don't need to lose weight, but I still don't feel comforable

: http://alcoholdrugstreatment.info/why-arent-i-getting-skinny