This may or may not apply. I had intense pain in my forearm while holding up a bass trombone. I got xrays, mri, physical therapy, massage, etc. The problem was a pinched nerve in my neck. Two things solve that - steroids, but that is only temporary, and the second is surgery. I use euphonium because it gives me relief from the bass bone ergonomics.

For bass bone, I use an ErgoBone device, which helps, but it's a bit awkward. On tuba, I use a bandolier strap, and it works, but I would recommend one of those NeoTech straps. It's a little fussy, but if it makes playing fun again, it's totally worth it.

Best of luck.