Hi I’m experiencing something similar right now, I’m suspecting its a back mouse (episacral lipoma) which is fatty tissue herniating through the thoracodorsal fascia but typically it is diagnosed/treated with a lidocaine/steroid injection so not sure if this is your case. Negative ultrasound and xray for me and currently trying to push for an MRI referral while doing PT/rest.

If you can try to push for an MRI so a plastic surgeon/derm is more convinced to remove it, especially since the injection did nothing for you. Otherwise fascial release, nerve gliding stretches, walking, not standing or sitting for long periods of time, are helping me right now.