Hunching over wouldn't, I assume, have anything to do with bone density. Have you ever been on steroids? What are your other lumbar scores? I seem to remember the average was -4.4-?

With the -4.9 I think you might want to avoid resistance exercises, lifting, or certain movements until you see the doc or a PT. Check out the melioguide (margaret Martin) site for advice on exercise: https://melioguide.com/

You are probably at high fracture risk. I fractured three lumbar vertebrae with one unwise movement and I was in the mid -3's. You can find out what you can do safely. Also not all PT's are good with osteoporosis.

I hope you have a good endo because this is indeed a mystery. Let us know! ps tai chi is good, but not yoga!