Might be a referral from the knee joint or even the proximal end of the fibula where it attaches to the tibia. There's a nerve (branch of the peroneal) that winds around the boney end just below and to the outside of the lateral joint line. But there's also nerve entrapments in the peroneals, referral from the joint line, other knee joing possibilities. Of course it could also be a low back referral but in the first instance I would check the knee, tibiofiular joint and the ankle as well. There's a lot of injury possibilities to that area. Some of them can be seen on the link below.

Proximal Tibiofibular Joint Injuries - Wheeless' Textbook of Orthopaedics

I'd suggest seeing if you can bring on the symptoms and if so then go see a good sports physio and show them what brings on the symptoms.