Tibia pain - stress fracture??

Hi. Posting this here in the hope of getting another opinion as my rheumatologist is stumped.

I have had pain in my left tibia since November 2019 which came on suddenly after a long walk. I haven't been able to run for 20 months and for most of the time, it has been painful to walk, often to the point where I have to use crutches for a week or so.

The pain is very localised, about 4cm above my ankle bone on the inside of my shin. It feels like there is a groove in the bone running diagonally across it where the pain is.

Everything I have read on the internet makes it sound like a stress fracture but 2 sets of xrays and an MRI were both clear. There was no soft tissue damage on the MRI but the xray did show osteopenia. I know I have that anyway but it has been quite stable for 10 years and I had a bisphosphonate infusion in March which made no difference.

Any thoughts what else it might be?

I had a very similar problem 10 years ago - unexplained pain in my tibia (this time just under my knee) which came and went and eventually spread to the same spot where I have pain now. That took about 8 years to clear up but I was then pain free. The doctor at the time concluded it was due to multiple stress fractures caused by osteopenia and possible migratory osteoporosis but he was never sure.