I had this a few years ago and it took percussive massage and stretching to get rid.
Prod around the muscles around your buttock and thigh areas to try and find a sore area.
If you find one then you can try a foam roller, or more likely something like a tennis ball, to apply pressure and ‘roll it out’, or have someone try to apply percussive type massage to break it up.
Be aware that it will be freakin’ painful.
Apply heat/cold/etc to the resulting bruising and do the stretches as mentioned.
If you can’t find a sore spot in the muscles it might not be piriformis syndrome so you might need to go see the doc.
I have just also had some muscles in my thigh go sore and had to roll them out, followed by quadricips tendonitus, which I have ‘rolled out’ with ‘The Stick’ – the cycle of smash up muscle and let the body fix it up seems to work quite well.