Probably the biggest worry would be a TIA (transient ischemic attack) but if it was a boiling hot day, maybe heat-stroke.
I agree with mgraylorn, if it happens again get to the root of it (probably a cranial MRI is a good idea if your insurance company is compliant.)
Fainting is NOT normal (well, not since women in the 19th century anyway!)
Test your own blood pressure at home and see what you get. If you don't want to buy a tester I'm sure at least one friend has one. Even the freebie machines in stores are better than nothing.