Drastically loosing weight?

I'm loosing weight extremely fast, there's been no change in my diet or exercise, i'm drinking the same amount of everything. I thought i might be ill so i spoke to my lecturer at uni who's a Dr himself and he said if i don't feel ill in myself then i shouldn't be to worried about it. Bloods have come back fine and everything. A lot of people over the past month have commented on how much weight i have lost and flat mates are getting concerned. Any idea what it could be? i'm studying medicine myself and have no reason to believe it's anything medical