Pink Parrot,
Welcome to the boards, I like your name!
Although I would certainly document it, and mention it to your doctor next time you visit...Your description of your fainting experience sounds very familiar.
I am a very experienced fainter... spent several years fainting daily following 4 strokes. Although the cause of my fainting were quite different than yours, fainting is a common reaction to a sudden trauma (such as your sprain), and results from a momentary loss of blood pressure to the brain. Your fainting spell is your brains way of pushing the reset button. In those seconds right before you faint, you get signals to stop talking, stop moving, fall down, reset, wake up.
Although fainting itself is a very natural occurrence, now that you have fainted, you know what to do. The real danger in fainting is hitting your head when you fall. If you ever feel faint again, just get down protecting your head.
Wow, this is the first time I have realized that I know so much about fainting. What a trip!
Good luck, and great description of fainting!