what do i do?

I'm 20 years of age and weaigh almost 100 lbs.I have been depressed for about 7 or 8 years and i eat a **** load of food and gain anything. one month i ate alot and sat on my butt for the whole month and went down to 80 lbs. i can i gain weight

[Roger Gould, M.D.]

This sounds more like a physical problem, than an emotional eating problem. If it were emotional eating, it would be that you eat more because you are depressed and you gain weight.  IF it is not physical, then it could be that you have what used to be called an agitated depression, i.e. that you are not only depressed, but very anxiious or "wired" and that makes  you burn so many calories. If that is the case, then ask your doctor about an anti-anxiety medication, such as klonopin.