Yes it has happened to me before, and it is quite common for some women. The main cause is that alot of your blood leaves major organs, such as the brain and digestive system and goes to your genitals. The position you were in can sometimes make these factors worse. Also did you eat a meal soon before? Or did you have an empty stomach? If you ate too much, and the blood left your digestive system, this causes improper digestion and nausea. If you didn't eat at all, you may have had no energy, and felt light headed or dizzy as well. Sex in the shower can also be a cause, because the heat makes your blood thinner. When you stand up (or if you are already standing up) it is easy for your blood to leave your head. This can cause fainting. That's what happened to me. It's not a big deal, just try to think of the reason why it happened, and avoid doing it again. If it persists, and happens every time you have sex, see a doctor because there could be something wrong.