Why could I have lost 20lbs in a month?

I am 16 and I go to my doctors every month, because I suffer from some insomnia. For the last month or so I have been it a real shi*y mood, I mean i know that I get mad for no good reason, but that dose not stop me from getting angry. This morning I work up and decided to weigh myself( and I my weight is now 138lbs) last month when I went to the doctors I weighed 155. I am 5 foot 10, so 155 is not over weight, when I looked at the scale I could not believe my eyes, i just lost 17lbs in 20 days, for no reason.(I did see if the scale was off, and it is not). I do not have a eating problem that I know of, I eat when I am hungry and I do not eat when I am not. I have been less hungry in the past month, but I still eat like I should. How scared should I be, I mean I know that parasites, can make you loose weight like this, but I do not eat fish, and i do not eat, ground beef to often. Now that I think of it about a week ago I felt really sick, with bad diarrhea and nausea, but that only lasted 3 days or so, what could make this happen, and what should I do about it, I plan to talk to my doctor about it next time I go, but is there anything that I should know for now.