Today I almost blacked out in the shower after having sex...why could that have happened?

I'm not really sure where to post this so I'm sorry if this isn't the right place. :/

Um, so I've only actually had sex like three times. I'm usually kind of tense before actual penetration but once it goes in then I'm fine and enjoy it.

Today I felt a little more pain than usual and was tense longer, I might have just been feeling pressure because I know that it usually takes a minute to get started. Anyway, we finally get going and it doesn't hurt but we notice that I bleed a little bit. I bled the first two times as well. After we're done we go and take a shower and my boyfriend starts telling me how I should try to relax more because my vaginal muscles tighten and it makes like a wall and the tenseness is probably why I bleed. While he's talking I start feeling a little dizzy and the more he goes on I start to feel more dizzy and a little nauseous and then my vision gets kinda fuzzy and dark and I start getting like a ringing in my ears and then I can't see anything but black even though my eyes are open. I stop seeing and hearing what's going on and my boyfriend sits me on the bathtub floor and after a few minutes everything goes back to normal.

Now, sometimes I get weak and a little dizzy when I'm hungry or haven't eaten in a while. We had sex in the morning before I had eaten anything and I lost some blood but it didn't seem like too much. I think it has something to do with my blood sugar but I've gotten my blood tested and my doctor said that everything came back normal. Once when I was at the optometrist's a similar thing occurred where I got dizzy and my vision turned fuzzy and dark when I was looking at an image of the inside of my eye and the doctor was explaining that I had a mole inside it, and I ended up putting my head between my knees and the dizziness went away after a few minutes. For some reason, I usually get queasy or dizzy when I see things about surgery or even hear people talking about surgery or body functions in detail (like hearing tendons, ligaments, veins, etc.) and I imagine it. I don't really know why I get so weird about it, but that's only happened once.

So...does anyone have similar things happen to them? Can anyone possibly explain why I nearly blacked out? Was it a combination of things and should I go to the doctor about this?

(Sorry this is so long, I didn't know which details would help or not)