I had a problem like this when I was on a cycle, my doc said my prostate was fine, however, when I got off my cycle it went away. I would get them when I would strain or jump sometimes and even taking a dump on occassions.
I personally think it was an unforseen prostate problem, maybe even an infection of some sort caused by a slight swelling of the prostate. My doc said it was probably a bladder spasm of some sort. But never occurred again so I never worried about it. My doc only did the "finger" test and I don't think that is a thorough way to check the prostate, and surely can not detect infections and other elments that can be caused by prostate problems. I'd go see a doc anyway and explain to him what is going on. If you find out do tell us here, because I am currious also about what is going on.