What could be the cause of my lower spine pain that just started? Use lumbar pillow when seated.

Spine Injury. This chronic pain in the distribution as you suggested is the result of an irritated nerve or facet joints or other injury typically in the lumbar spine (low back) which are caused by herniated disks, spinal stenosis or degenerative disc disease, etc requiring further evaluation by a spine specialist.
Let me try. If you started to use the pillow recently and you do not have back pain before, most likely some posture adjustment.