Check bike setup first. Perhaps put some EMG (electromyography) pads on the quads etc and have them ride on rollers. Then measure what's happening where as you change things like stem height, seat position and height and distance from seat to stem.

All these have influences on what happens to the leg during cranking and therefore the potential overuse of one side, including the piriformis. You must check out the amount of internal hip rotation in flexion. Note this is lessened by an increase in stem height.

It might sound like a lot to consider but it really works. please let us know how you get on.