Sounds like it could be your sacroiliac joint

I have really tight hips and hamstrings (I stretch every morning). About 15 years ago I was doing squats and I slightly dislodged this joint. I was having pain in lower back and some numbness. Really bad pain anytime I sat down.

I saw a back doctor and he actually recommended back surgery because I had a mild disc herniation too. Had some injections into lower spine but they didn't work. Come to learn, when I went to PT, that my pelvis was slightly out of alignment as the joint had slipped. The poor alignment (tilt) caused the slight disc protrusion to hit my sciatic nerve running down my spine cause great pain. When you sit, it exacerbates the disc protrusion so it hits the nerve more causing more pain. He adjusted the joint back into place and it takes a while for the inflammation to lower (depending how long it has been out of place). Pain completely disappeared. I have had the joint slip a few times over the last ten years, usually with an awkward movement or lifting something heavy.

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RIP Bucky - 7/25/1943 - 1/19/2013