Why the hell did I faint?

I know this isn't the best place to ask, but I'm curious to know if this was "normal."

I was talking with friends and my arms suddenly felt very, very heavy and my neck started to hurt. I didn't think much of it. Then, it felt as though something was squeezing the back of my head and I started to feel nauseous and contemplated excusing myself to go to the bathroom so I could throw up. I was looking at one of my friends, trying (unsuccessfully) to listen to whatever story she was telling, and my focus started going in and out. My vision got a little fuzzy, then it was blurry, then too blurry to make out my friend, and before I knew it, I had fallen over and was sitting on the floor, against the wall. My friends gave me water and I felt a little better (the nausea went away, I guess).

I know that I fainted, but the squeezing feeling in my head persisted and I've had this headache that comes for a few minutes and then disappears for awhile ever since I fainted (I'm not usually one to get headaches).

I don't know why I fainted though. It was a little warm but not oppressively hot or humid (it's was a rather warm day for where I live, but it was no warmer at my friend's than at my house) so it couldn't have been the heat. I wasn't hungry, having had a big lunch today and I'm not under any stress or feeling anxious. All I can think of was that it started with my arms feeling heavy and my neck and shoulders aching.

I've never fainted before and am rather spooked as there seems to be no reason to have fainted; I wasn't overheated, too hungry, I have no medical conditions, and I'm not on medication.