Three things come to mind.
1) Not breathing fully and deep enough or something might be restricting breathing (lifting belt comes to mind).
2) Nerve or muscle cramps/stress, which can relieved by massage or stretching. Usually occurs at the neck, trap or upper back area. In rare events, lower back.
3) High blood pressure. Which really is a silent killer. Pro BB'ers and PL'er seem to have this problem quite a bit. My race and blacks have this concern at a higher percentage rate than whites or any other race.
If the light headed feeling is not felt that much with higher rep's, that might be because your using a lighter weight as opposed to using lower reps, with more weight used. And more force put into the lift. Which might hint at higher blood pressure. Don't have to be some old guy, fat or out of shape. Now days even teenagers can have HB. Girls included. Might be surprised how many prime athletes have blood pressure problems.
There is a distance #4 reason, which may be a stretch, and that would be heart disease. The heart is not pumping enough blood/oxygen through the body. Blood flow is not regulated the way it should. Would doubt if it would all come down to that. Anyway might check the family history and your blood pressure from time to time. Good Luck.