Popliteus is the most likely culprit.
Muscles that criss cross joints obliquely tend to get tight if you're compensating for something... and you would be compensating from the glute strain.
Since all of the fascia is connected on the posterior chain I would suggest rolling your plantar fascia, stretching calves, and hamstrings. Soft tissue work to those areas would likely be good as well.
Also, this may help: