Tonsils, knees and emergency rooms

We had some fun recently when both boys had their tonsils taken out. They both continued to get strep over and over again. Let's just say that in the prior six months, they had strep more often than not. So we made the decision to get their tonsils out, and decided to do them both at the same time.
What a challenge that was, but we wouldn't change it if we had to do it all over again. For the most part, they seemed to alternate not feeling good afterwards, so it was never both really cranky at the same time.
But the day after the surgery, we had an ordeal where Colin complained about his knees really hurting and he couldn't walk. He did this for over half the day and his fever spiked above 104 and wouldn't stay down below 102 for more than an hour or two before it would spike again. Luckily my parents were here and Tammy and I took him to the emergency room, where we spent the next 9 hours as they did all kinds of test on their "mystery patient of the day."

What parent wants to hear a doctor refer to their kid as the mystery patient, due to his symptoms stumping the doctors? Not a good feeling at all. They did blood test, xrays, urine tests, flu tests and I can't even remember what else. After about 9 hours, the doctors were still stumped but Colin was starting to feel a bit better and could stand on his legs, although he still didn't really want to walk. They gave us 4 possible diagnosis for the symptoms: 1. Some reaction to the medication or anesthesia from the surgery. This makes me curious what they used this time, as Colin had his adenoids removed and ear tubes in a prior procedure (same doctor). Plus, why the knees? It doesn't make a lot of sense. 2. A virus that sometimes occurs in kids that age that they call some kind of "transitory" virus that causes inflammation and pain in the joints. Maybe, but quite a coincidence on the day after surgery to get this transitory virus. And again, why just the knees? Why no other joints like the shoulders, ankles or elbows? 3. Regression - sometimes kids regress after surgery. I have a real hard time believing this one. I could see peeing in his pants or acting like a baby or some other things to get attention. But to go for over 19 hours complaining about his knees hurting without even once forgetting and walking/running before he remembered. He's only 4, after all. I couldn't even remember that long. 4. Position of the legs/knees during the surgery. This is most plausible to me, as it's possible they strapped him a certain way during surgery to hold him steady and maybe the day after surgery when the pain medication wore off, he was really beginning to feel the stiffness and soreness from how they positioned him. Even so, nothing was certain. But the next morning when he woke up, his knees seemed fine and he was walking okay. What a relief! Still, it would be nice to have a definite diagnosis, especially if surgery were ever needed again. Was it something they gave him during surgery?