I have a pain in the middle of my back that feels like a bruise. i have had it for a few weeks and i didn't hit it or anything.

Needs examination : Can be a variety of things may be back pain or may be pain referred to the back by other structures (eg pancreatitis kidney infection). Needs examination and may need tests to sort out. See your doctor

Examination: Before knowing how to treat your back pain the cause of the pain has to be determined. May be from trauma, localized neurologic-skeletal malrelationships, referred pain from other organs, etc. Please see your Physician.