What's "normal"? It may not be common, although I have read that many people -- estimates as much as a third of all men and women -- get off at least sometimes through pain-related fantasies. And a fair number of people enjoy at least fantasies of pain or dominance or both in sex play.
What I find more interesting is the question of why. I suppose, in the absence of information to the contrary, that people just early on, as children, associate pain and genital stimulation. But who knows? In any case that was why I asked the question of tittygirl. And she gave an honest answer, though I was a bit surprised she started masturbating so early.
Tittygirl -- did you actually climax at the age of 8 when you masturbated? Do I read you correctly that you must have a fantasy (as opposed to the reality) of being in a painful situation in order to get off, regardless of the other stimulus?
And by the way, I can empathize with you, because I often find fantasies of being submissive or dominant exciting. But I do not know why I do. I started having them, sometimes...but not all the time, when I began masturbating at about age 11 or 12 (to climax).
I also find medical fantasies, wherein I am invariably in the submissive role, very stimulating, but they don't involve pain as much as a type of exhibitionism. But I think I know why in this situation -- it is because I had several, quite sexual encounters with nurses during hospital stays just before puberty and through my teen years.