I had high blood pressure passed on to me from an entire family on medication. Whenever I do squats I get light headed and then feel sick, even at low weights. I can do sets of 10 at 125, anything above makes me feel like my hearts going to explode or I'm going to pass out. This sucks considering I'm a pretty fit 6'3" male at 220lbs.
Just to give a comparison, the rest of my body is pretty strong. I can calf raise a couple hundred lbs and I don't feel the same effect at all.
Squats for sets of 10 are pretty hard, probably why you are getting so lightheaded.