For about a week now, i've been experience a numbness in my mid back to the left of my spine. especially when sitting. what is this?

Pinched Nerve: Sounds like you have a pinched nerve. If this doesn’t improve or gets worse we your doctor

Spine or muscle:: This could be due to a "pinched nerve", from disc disease, or muscle spasm compressing a nerve. There are other possible causes as well. After a week of symptoms, that have not gone away, it's time to get evaluated to find out what is going on. A mass of some kind can also compress a nerve, so it is best to get checked now. If money is an issue, searh: low cost/free clinics on google or CA govt