Just like to add this:
From my experience, at least 80%, maybe more, of people who've recently had ANY kind of surgery, will, a few days later feel that something is TERRIBLY wrong.
My theory is that all the initial anesthesia has worn off, now getting PO pain meds, as opposed to IV, and maybe most importantly, they are no longer the center of everyone's attention. I truly don't mean this in a bad way. It's just that immediately post op, there is a ton of monitoring and close contact with the health care team. When this eases off, the patient/family starts to take over the worry. Then, I think the anxiety actually increases pain, and here they are. It happens in peds ALL the time. I mean so frequently, that it's almost comical.
As a side note, my own contribution to allaying this visit is to go over discharge instructions VERY carefully with post op patients/families, so that they are very clear about what to expect and what to watch for.