Do you get any similar pain while attempting any demanding stretches that will require high flexibility and mobility in multiple areas, e.g. front, side, middle, pancake, wall splits? These should hopefully isolate a specific area and indicate a mobility issue.
Or how about pain while doing the "hollow body" or bodyweight versions of deadlift, squat to parallel, ass to ground squat/goblin squat. These will also isolate an area, but focus more on identifying imbalance, strength than mobility.
Two final suggestions for simultaneously possible immediate relief and problem isolation:
go to bed and wait 5-10 minutes as your body relaxes and you almost fall asleep, then remove any/all your pillows. Try the problem position and see if the pain has reduced or disappeared for a night.
grab a sleeping bag or move your mattress from your bed and sleep on the floor for a night.
If none of these help, then the issue is likely to be tightness in hip flexors and possibly glutes. Exercises like runner's stretch, lunges, heel-to-buttock stretch against a wall, yoga's Pidgeon Prep or box stretch should all help considerably over a few weeks.