Why don't I feel like eating anything?

I'm 18 going on 19 and I'm 5'2, 91 pounds. For the last two, three years my average weight was 97/98 pounds. I'm NOT anorexic, nor do I have ANY eating disability. This is VERY common in my family. I eat all of the time and I actually eat very unhealthily. I eat a lot of protein (meats, eggs, peanut butter, fish, etc.) a lot of sodium (I like to over-salt everything and drown things in soy sauce) and a notorious sweet tooth. I've always loved food. I eat all of the time and in surprisingly large amounts. I just have a very fast metabolism that's common in my family (my mom had it until she was pregnant, my dad had it until he grew older, etc.). I actually have trouble gaining weight, which I feel would be much healthier for me.

Thing is, lately I haven't had the urge to eat. This has been raising eyebrows within my family and friends. I mean, you know that feeling in the early morning when you don't feel like eating because your gag reflexes act up? Well, I feel like that all day. I have to force myself to eat. I get hungry and I fix myself food, but when it comes to eating the food, I just lose my appetite for some unknown reason. I don't even feel like eating sweets and meat. I'm worried because I just lost like 6 pounds in a matter of two weeks and I'm already considered underweight. How can I regain my appetite?

I should note that I don't exercise much, at all, and I have a kind of messed up sleeping habit. My friend says it might be my sleeping habit.