16 year old painful back pain, upper middle. no other sympts, occasional lift heavy goods. but haven't for 4. weeks, pain started today morning.?

This is most: likely a muscle strain. Have you worked out at all? It could be from that, even if you haven't lifted anything heavy, possibly from twisting in such a way that a muscle became strained. It also could be from sleeping in a certain position. You could try taking ibuprofen if you have no medical contraindication, but if it persists, please see your doctor.