constant heavy feeling, lightheadedness, fatigue, symptoms worsen with menstration

Hey,

I am a 19 yr old college student who is a non-smoker, light drinker on occasion, eats very healthy and works out on a regular basis. I try to get between 6-8 hrs of sleep every night. For almost a year now, I have had a constant feeling of being "heavy", literally, my body feels extremely heavy! I have had difficulty lifting my head off of the pillow on occasions. I get lightheaded from time to time and seem to constantly "zone out", like I can't make myself focus. It's as if I'm just kind of there, not really consciously there. My symptoms worsen dramatically the day before I start my period, I am completely lethargic. But during menstration, I am completely fine. I have had numerous blood tests done and everything checks out. It's not anemia or hypothyroidism or diabetes, and my blood pressure is within normal limits, even on the low side sometimes. My hormone levels are normal as well. I am not depressed or overly stressed out, I am a very calm person. What else could be wrong with me? Can someone just give me an idea so I can research it further and talk to my doctor about other courses of action? I would appreciate it greatly!!