[UConnJulie]

Backs are tricky sometimes ... it could be as simple as a facet joint strain that has turned chronic, or it could be a mild disk injury (prolapse or bulge versus frank herniation). Doubtful that it is muscle as muscle heals relatively fast and would not still be bothering you at this time. There are just many structures in the spine which can give similar pain. Honestly your best bet would be to get it evaluated by a competent professional in person and treated accordingly.

But, in general, if it hurts to sit, you will likely respond well to backward bending exercises ... like lying on your belly propped up on your elbows (for up to 5 minutes at a time), or a "press up" exercise (like a push up but leave your hips on the ground in sets of 10, 2-3 sets 2-3 times daily).
Make sure your hamstrings and gluteals are flexible (if you sit for a living, they are prone to poor flexibility). When you sit, because it is unavoidable at certain times, then sit in good posture with your tailbone not in contact with the chair (just your "sits bones"/ischial tuberosities) imagining yourself being lifted up from the crown of your head.