Fluid in endometrium

Last year I had fluid in the endometrium that was found incidentally when having an ultrasound and CT scan for kidney stones. There was no free fluid and the lining wasn’t thickened but they drained the fluid and had it biopsied anyway it came back negative and nothing more was said.
Last week I had another ultrasound for kidney stones and fluid was found in the endometrium again. Now I have to go back to the gynecologist as my doctor recommends. Has anyone had this happen? Why is it happening? Should I just ignore it because I would’ve never known this was happening had I not had a test for another condition . Is this common after menopause? I am totally confused by the way women are treated after menopause. Told internal exams are no longer needed. I don’t know if doing so would have revealed this sooner. Any help appreciated.