I have a similar pain. It starts high on the back of the (right) leg and goes up into my butt. It doesn't hurt when I run but certainly is tight and hurts after... especially after sitting at work for a few hours. As soon as I get up to walk, I have trouble taking a step forward with the right leg. I've tried stretching by laying on my back with my right leg crossed over my left knee and pulling the left knee towards my chest. I've also done this while standing. Cross your right leg over your left knee and sit - helps to be holding on to something like a table or a chair. I don't know if this is helping yet, but it feels good. (the stretch was recommended to me by another runner) If anyone has any idea what is wrong or has any other suggestions for what to do, I'd love to hear them too.