The way you've described the pain sounds like it's tension in the small muscle there. Like your tensing it ever so slightly and over time that's building up, a little like it's easy to hold your arms out horizontally and then after a minute or so it starts to ache and then really hurt.

I'm no doctor and I may be barking up the wrong tree, so apologies in advance... Next time you go to the bathroom stop yourself mid stream, if you can, do it a couple of times. This will exercise that muscle and maybe build it up a little. Try doing that for a couple of weeks. Also if you start to feel the same sorta pain after a few goes, you'll know that your on the right path.

Again, sorry if I'm off on the wrong path, just thought I'd suggest it.