Hmm, I think the exeternal rotators are actually lenghtened in internal rotation?No?

This may also be a bit of trochanteric bursitis from sustained load on the ITB against the greater trochanter of the feumr when adopting that position. i.e. a chronic injury caused by the annoyance if the sitting position. Get someone to test that hypothesis on an assessment and you would try a form of saddle seat to stop you assuming that position for a while to see if that assists to reduce the symptoms.