I posted this message a while ago to someone with similar pain, and it may be relevant:
"From your symptoms, it sounds like piriformis. One easy way to tell - get someone to push their thumb as hard as they can into the middle (usually the softest part) of the affected buttock. If you don't hit the roof in excrutiating pain, the chances are it's NOT piriformis syndrome!
The piriformis muscle is intertwined with the gluteus maximus, and the sciatic nerve runs through or around the same area. If you have a biomechanical problem, the muscle can become compacted, which causes the buttock, back and leg to stiffen and become sore. Simply stretching the area is not the best way to start treating it, as stretching will pull the edge of the muscle only, weakening that part and causing the central part to compact further to compensate.
My chiropractor used to loosen off the core of the muscle by leaning all his weight on his elbow on the area, and moving it around. At first I thought he just didn't like me, but eventually (after about 3 sessions) I could feel a difference, and he then gave me some stretches to do, which, when I am diligent about doing them, really help keep it at bay.
The same effect can be had by sitting the sore area on a tennis ball and gradually rolling it around the buttock. Just as painful as the elbow, but at £1.50 for 3 tennis balls compared to £25/half hour for the chiro, much less painful on the wallet. Once you have done this a few times, lie on the floor with the affected leg bent and the other one straight. Pull the knee of the affected leg towards the opposite shoulder, gently stretching out the buttock. The chiro had me doing 3 x 30 seconds 3 times a day for the first month, then 3 x 20 sec x 2, then 3 x 20 once a day for maintenance.
I still get the odd twinge, but I'm sure if I kept up the stretches, I wouldn't."