Hi - sorry to hear about your cancer. are you sure they were checking your actual levels and not just checking to be sure you are taking some pills and not selling them? Levels in urine tests are very expensive and not often done. At any rate - when is your next appointment? Take your medication as needed for pain, don't take extra because while you're obviously opiate tolerant, you wouldn't want to accidentally OD.
I wouldn't try to "catch up", as that would be really dangerous. I would just be honest with the doc and tell him your concerns.