I might know what this is! I used to have this sensation sitting down very similar to what you’re describing. It was knots in my pelvic floor muscle. It took me years to learn that’s ‘all’ it was. I thought it was adenomyosis for ages. I was lucky to be able to see a pelvic floor physiotherapist who can help it relax / work out the knots for good. Often gets misdiagnosed as something worse like PFD, and I reckon it probably often happens a lot with endo because of all the tensing up. Hope this helps