Can't gain weight (5'11 115 lbs,) parents think I'm starving myself to death?

19 year old male.. a sickly 115 lbs at 5'11... I just CAN'T gain weight no matter what I do. There we periods where I would binge eat an entire pizza, pounds of chicken, an entire case of donuts for days on end and not gain a SINGLE pound. I just gave up trying to gain weight.. and now my weight is decreasing steadily and I can't stop it, it seems. A few weeks ago I weighed 130 lbs, and now I weigh myself and I presently weigh 115.. my diet hasn't changed in those past weeks! I don't exercise, I don't do anything to burn calories.. I just sit around and I "LOSE" weight. Hell, there are people out there that probably wish they were in my shoes.. but I am WITHERING away.. my parents think I have some sort of emotional problem.. I DON'T. I just can't gain the god damn weight and it keeps sloughing off of me like dead skin.. it's freaking scaring me to no end. Where is it going to stop? 100 lbs? 75? Christ, I already look like an emaciated etheopian as it is, WTF IS WRONG WITH ME.