Why can't I eat as much as I could a few months ago without getting sick?

I'm a 21 year old male and I need help. A few months ago I was doing weight training and I was putting on much weight but all of a sudden one morning I just couldn't eat. I thought it might just be a stomach ache or something and I didn't worry too much about it. I got worried about it after it didn't go away after a few weeks and I was throwing up from food about once a week. There are heaps of symptoms but they come and go. I saw a doctor about three times and I had blood tests and X-rays but they didn't find anything. I have lost 7 kgs (about 15 pounds) and I was a small guy before this too. I don't know what's wrong because I used to love food and now I can't stand the thought of it most of the time. If anybody can help I would appreciate it.