I want to know why I'm always lightheaded.?

Every time I sit down, get up or walk a few feet after doing that my vision hazes, I get lightheaded, and get very weak. I nearly fall down most of the time. Also, I don't eat a lotta food, just dinner and occasionally breakfast. So, I wanted to know if it was because I needed to eat more or some other reason. If ya know why, could you please tell me? P.S. I'm positive I don't have Diabetes, so don't say that.