What Causes Headache And Lightheadedness?

I'm constantly fatigued, tired, sometimes get a headache, or random pains, very lightheaded, I've lost around eight pounds in the last, maybe, three months, for no reason, I switch letters around, can't think straight, and I can't see well. Black and white text turns some letters green occasionally at a certain size. Could this be cancer? Or something minor, and I'm just overreacting?