Hmmmm... If Ken's hunch is right about the piriformis:

Several years ago, I had a hip/butt problem, but it was pain while driving long distances ...right leg, aching hip/butt area. I asked my doctor, and he was sure it was arthritis. I told my physical therapist, and he thought I should try a piriformis stretch.

I started doing the stretches -- 60 seconds on each leg -- after each workout, and the problem has completely disappeared.

Here's a picture, from this website:


And a picture from this website:


When I do the stretch, my knee is centered left-to-right between my elbows.