eomahoney, please see my response to a similar problem in a thread named
"Overuse groin injury with very specific cause"
. Your problem is almost certainly a spinal referred one. Neuralgia ( nerve pain ) and other altered neural effects would most easily and logically answer the Why part of your dificulty. The What to do part is also covered in the other thread. In adddition , having had an observation of pronation ( 'flat feet " ), puts further points on the likely cause being your lumbar spine. Suitably made and fitted anti pronation orthoses would be beneficial in eliminating what would be an ongoing biomechanical contributor to spinal protective responses causing your neuralgia.
Cheers.