Have you asked too if relationships of other kids have changed or if there were changes with other kids? DS3.5 out of the blue told DH he did not want to go to school one Sat night, and although we are not sure what drove that, one of the major changes we noticed was suddenly two of the classmates developed an exclusive friendship, and one of them even yelled at DS to walk away at drop off in front of DH. That exclusivity between the two kids wore off, but I suspect due to his emotional sensitivity and increasing boredom, that when that incident happened, it was the straw that broke the camel's back.
And at this age, they may suddenly realize that they are no longer in sync with their classmates(we know DS was holding back and his teacher had no idea that he was bored - but as soon as we moved him to pre-K, he found friends he loves - in the K room (pre-K and K will mix and play regularly throughout the day)).
He definitely could not tell us that he was not on the same page with the other kids, but in retrospect, it looks like his sudden change was his realization that the repetition (which normal children need) was getting too much for him, and he could not even count on the kids who used to be his friends to stave that boredom off... we only realized this afterwards that it was most likely the combination of multiple factors (no new toys, material repeated over and over, no friends at the same level).
