What does it mean when my arms and legs feel weak, and I feel light headed?

This happens to me occasionally. I take medication for epilepsy, and am also taking some meds for a nasal infection. I'm not super skinny, and i'm not seriously overweight. I think I could work out more though, and burn off a little fat..i'd say I'm average I guess. I also have a small lump in my breast cleavage and lately I have been worried that it could be breast cancer..