I had what I thought was Piriformis syndrome. It manifested in a sharp pain in the left ass cheek, right under my sitbone, about 30 seconds after I'd sit down (while riding). It was quite painful! It started in a stage race last July and kept going.

I stretched and worked on it and got it sort of better but it still hurt. Last week I realized that when I sit at work I cross my left leg over my right. All day. I stopped doing that and suddenly the pain disappeared.