I think i slept in an awkward position that caused pain at my back.got worse when i carried my bag to work. now, really hurts when moving or breathing.

Heat, massage: You may have some muscles in spasm. Using heat or massage, perhaps with a muscle relaxer medicine, might be a good place to start. Other causes have to be considered if you don't respond.

GET IT CHECKED: I think you probabely have a sprained back just rest and take over the counter meds like aleve (naproxen) or Motrin and if it does not get better see your doctor.