My reaction to pain is just weird. Acute injury--cuts, bruises, burns, that kind of thing--doesn't faze me. Half the time I don't notice it. It causes pain, but not distress. On the other hand, things like a headache or menstrual cramps can make me feel dizzy, pass out, throw up, get drenched in sweat and pale; be unable to stand; generally react as though I were in extreme pain. My heart rate slows down when I'm in that sort of pain, probably due to the release of endogenous opioids.
What I learned in psychology class about pain is that there are two components--the physical damage and the message passed to the brain from the site, and the emotional reaction to the pain. The emotional reaction is there to cause you to protect yourself from further injury; it's the instinctive interpretation of pain as negative. Apparently, I don't have this emotional reaction for some kinds of pain. Other kinds, I feel very strongly.
BTW, by "emotional reaction" I don't mean what you'd think of when you first think of emotion; you know, sadness, frustration, anger. I mean the interpretation your mind does when it gets the "pain" message, the one that says "this is bad, I don't like this," the thing that makes pain an aversive stimulus. For some kinds of pain, I just don't do that.
I have a weird sensory system; pain sensation is no exception.