1. probably you are talking about Methadone
analgesic] which is used as an analgesic to relieve pain by blocking nerve/pain receptors and at the same time substitution therapy of opioid dependence.
2.it can be possible that due to lowering of drug in the blood, it have unmasked the pain.
3. If no improvement occurs within ten days after the injection, then it is unlikely to gain any pain relief from the injection and further diagnostic tests may be needed to accurately diagnose the pain.