Is it on your right side? The piriformis does mostly external rotation, so if you use a lot of hip rotation during your FH stroke, assuming you're a right hander, then it would get stretched a lot during the back swing and activated strongly during the forward swing. Conversely if you're trying to use your hip rotation on the BH side.I had some of what appeared to have been the latter, but not anymore. That passed in two days or so. It could have just been the whole leg and back being sore, though. I had something similar when I started doing standing work in work boots for the first time; could barely walk after I came home. Maybe it's something similar this time; typical fatigue. Either way it passed.
I do have a bit of a muscle knot kind of feeling in my right cheek, so there's something there. It's not super bad right now. Stretches did work.
If I was younger and less responsible I'd hit the club in a day or two, but I know now it is wiser to over-correct the treatment a bit just to make sure. Sit on it a few days (hehe) without any symptoms and keep doing rehab.
Do some stretches (don't hurt it too much during stretching!), rest it for a few days (maybe do BH only practices?), and if symptoms are worse than mild then take Ibuprofen 3 times a day for a couple days and you'll be good enough to start playing again without worries.