I'm almost sure this is a hip flexor strain - I felt a pain when I was catching the bar at the bottom of a clean over the winter, definition of a sudden contraction under too much load.
Just weird that it's been months and still hurts.
I had a tear in that area 2yrs ago. Physio made it better but it took quite a while. I still battle with that side getting tight almost daily and this yoga pose is the most effective way I can get it to release.