Bad timing
Today I removed my final pain patch. An hour or so later, I was in pain! I hadn't felt any pain since day 1 post-op; had the pain patches been protecting me from this much pain the whole time?
Then my stomach started producing tons of foam. For the next four hours I belched and regurgitated the foam, and the pain lessened. Gradually I realised that the cause of all this was probably the Actimel that I had tried for the first time a that day. I am lactose intolerant, but I hadn't had any issues with yoghurt. Perhaps probiotic drinks are more milky? Eventually the foam stopped; I took a nap and woke up feeling better. No more pain, no more foam.
I'm glad to know that I haven't been in pain like this all along, with the pain patch masking it. It was just bad timing that I took my last pain patch off before going through this experience. I think I'll try probiotics in tablet form in a day or two.