What Causes Discomfort In The Tailbone And Rectal Area When Sitting?

Hi, I'm a 40yr old male. I have had a dull pressure/discomfort in the tailbone/rectal area when sitting on an office chair or any flat hard surface for about 3months now. The pressure/discomfort is not present when I'm standing up. My primary checked my prostate & it was not inflamed. No rectal bleeding, no blood in stool. I'm not sure if sitting on an office chair for 10yrs caused this or because I lifted a window AC unit from the 2nd floor to the basement & 2 weeks later, I started feeling this symptoms. Thanks for any advise you can provide.