For The Past Three Days I Have Been So Tired

For the past three days I have been so tired I can't keep my eyes opened. I went for my routine neurologist apt and my blood pressure was a little low...98/60. It usually runs about 110/70. I kept falling asleep in the doctors office, and they wouldn't let me drive home, I left school early today for same reason. I feel as if my body is so heavy. Its not as if I'm not getting enough sleep, I take a nap and wake just as tired as when I first went to sleep. No other symptoms....just extreme fatigue.