I Experienced A Marked Increase In Left Leg Pain When

I experienced a marked increase in left leg pain when prescribed oxymorphone 10mg er bid with oxymorphone 5 mg fast acting for breakthrough pain. Previously I had been on Percocet 10 mg qid for an extended period of time. After 3 weeks I had to stop because of the extreme increase in pain. Why would that happen? It should be hitting the same receptors in the brain, right?