Charlene,
Assuming you're in a power chair, I would change the tilt angle (starting at least 10 degrees) every few minutes and see if that helps, and likewise the foot height (more angle than tilt). If you're up to taking a selfie in your chair, we could offer other suggestions for positioning.
Are you in a default or upgraded back/seat cushion? The ones that come w/ power chairs are generally suboptimal. If it feels like a car seat, it's not redistributing pressure as it should.
Since you are using your arms a lot, it is possible that you injured/inflamed the muscle there doing so since it (the muscle just under the breast bone) is normally not strong skeletally anyway but your core will try to engage when you try to bear more weight on your arms than they normally handle. Have you tried your favorite NSAID (naproxen, ibuprofen), dosing it regularly (not missing doses) for a few days?
Also it seems counterintuitive but you could try a flexible heating pad in the small of your back. And massage, of course. You can get someone to come to you, at home, if getting out is a challenge right now. If you have home health benefits, you could ask your clinic about writing for a PT to come and assess you since you may need a better range of motion regimen to avoid what you are experiencing. And how are you transferring right now?
I'm sorry you are in pain!
--Laurie