It seems to happen to me depending on the pants I'm wearing...sometimes there is more of an opening in the zipper then others. For me after shaking and zipping up...it seems like the bottom part of the zipper area was putting pressure against the penis...when the pressure is released there is a bit of spotting.

If I no it happens to a particular pair...I usually will sit in a stall...

I would suggest getting it checked out like the advice given...