I would agree it likely has to do with blood pressure. I have been light-headed after taking Viagra (this is a side effect of Viagra). I think Viagra has an effect of blood pressure. I would get your bloodpressure checked. It may also somehow be related to change in hormone levels during erection. I have never heard of just an erection alone (without medication) causing light-headedness or fainting... I would get it checked out medically.
it must be because you have a realy big dick. it takes so much blood to give you a hard on. that you loes to much of the blood in the rest of your body. causing you to get light headed. just wondering how big is your dick hard anyway?