Tired

For some reason, I've had NO energy the last three days. I don't know why. I've gotten enough sleep...that's all I want to do. As soon as I start to do anything (even going up the stairs!) my heart-rate jumps and I start to sweat. I don't know if I have some sort of bug, or if I'm just SO out of shape....maybe I'm lacking in some vitamins? Any suggestions?