I had the same issue and went to Physical therapy. It was my iliotibial band that ran from my hip to down under my knee. He used massage, ultrasound, stretching and strengthening for it. It took about 6 mos. to make it feel better, but it still will tighten up, so I go back to get it worked on.
I had the same issue and was told that I needed to stretch nightly before bed. I no longer have any pain.
Good luck