Is something wrong with my blood?

When i work out my head hurts and I get kind of light headed. If I stand too fast I also get dizzy and nauseuas. Last year i kept passing out so i went to the doctor and they couldnt find anything wrong. I also can't give blood. When i try i can only get out about 80 ml and then my blood stops flowing. I wouldn't worry about this except for the fact that I'm an athlete and my seasons coming up. I was wondering what could cause this and how can I lessen the effects? (And trust me i'm super hydrated and eat good complex carbs before I work out.0