I'm so sorry shellmos - how hard it must be to hear your kid say that.
Our DS4, in his 2nd year of pre-k, had been saying he hated preschool for weeks, and i finally i asked the preschool teacher and she told me of an incident. (Apparently, he was balking and doing things that he already knew, and she finally said, you still have to show me, and that's when the hating preschool started.) We're still debating on whether to take him out of pre-k after christmas - it seems to have run its course.
Anyway, lately he has switched over from I hate preschool to "i hate this family." This happens whenever we have him do something he doesn't want to do - like hurry, e.g. So for our kid, the comments are definitely related to having to do something he doesn't want to do.
Maybe you can talk to the teacher to see if she knows of a recent trigger? Good luck!
