Bp was 132/74 before walking on treadmill 20min. drank 32oz of water while walking. became lightheaded and blurred vision. took bp it was 114/99. why?

Blurred vision: I do not doubt that you have a low BP as you describe. It does not seem likely it was due to a volume deficit, unless your were in a very warm environment.. Did you notice if your pulse was fast? If it was fast, was it regular? If this happens again, you should consider consulting a cardiologist to evaluate your cardiac rhythm. Good luck.

Check electrolytes: Drinking 32 oz water in a short time could caused electrolyte imbalance.