Its the singlespeed. It is slowly turning you into a complete animal. Be carefull.
As far as pain goes, here is a quote posted often in other forums I don't know if it has been posted in MTB though....
"To be a cyclist is to be a student of pain....at cycling's core lies pain, hard and bitter as the pit inside a juicy peach. It doesn't matter if you're sprinting for an Olympic medal, a town sign, a trailhead, or the rest stop with the homemade brownies. If you never confront pain, you're missing the essence of the sport. Without pain, there's no adversity. Without adversity, no challenge. Without challenge, no improvement. No improvement, no sense of accomplishment and no deep-down joy. Might as well be playing Tiddly-Winks." -- Scott Martin
I know most MTBer's don't consider themselves cyclists, but I do.