Amazingly, for the past week I have had the same problem. I have used the computer daily for nearly 30 years without any problems, but suddenly, last week I experienced a sudden stiffening of my back with severe pain both in the front rib cage and in the mid-back vertebrae. I've had an MRI which basically came up with NAD (no fractured vertebrae or ribs detected) - although I do have a history, since childhood, of fractured ribs due to spending my childhood on an orchard where DDT (now banned) was in frequent use. Osteoporosis was also detected in a bone density scan in 2018 with a Femoral Neck T-score of 2.8 and a Lumbar Spine (11-14) T-score of 3.4. And yes, I DO tend to have poor posture - both due to the above, and ongoing.