Can I buy a narcotic pain reliever without prescription while in Canada? and am I allowed to bring it back ?

Canada has very strict laws about the possession of narcotics, just as the U.S. does. You MAY be able to get a prescription for a narcotic pain killer while you are up here visiting, but I suggest that you also get a note from the doctor explaining why they were prescribed, so that you avoid hles when re-entering your own country. And for God's sake, don't try to hide their presence. If you do, they will be confied and destroyed.