Hi
In my experience, this pain is most likely to be Sacro-iliac in nature.
If pain also spreads to anterior thigh, or groin, plus erector spinae/piriformis is tight as a protective response, the Wikipedia reference-linkSIJ should be treated. Reduce the muscle tightened areas, teach exercise for rythmic alternating hip raise in supine for two minutes. Chronic problem may need narrow 50mm SIJ rigid belt, to be applied and tightened when in supine - only use the belt if not responding to other therapy.
MrPhysio