Why am I always sleepy?

I am ALWAYS SLEEPY!  I am in college and over winter vacation, I would sleep 12+ hours a night and still be exhausted when I woke up!  Now I get 6-8 hours a night, and I am tired all day.  I'm physically weaker than normal too.  I'm too tired to practice music, too tired to exercise, too tired to walk my dogs, and too tired to have sex.  Increasing my caloric intake to 1600 calories a day has not helped.  I can not drink coffee b/c I have IBS. Caffeinated Mate Tea works for 2 hours then tapers off and I'm exhausted again.  

Why am I so tired?  Does anyone have suggestions?