Kids at that age can be very sensitive around their peers.
My own son is steady as a rock most of the time (he is 8) and then every once in a while...I say something and he disappears. I go looking for him and he is in his room crying and says I hurt his feelings or was rude to him when I don't even realize anything bad was said/done. It makes me feel awful for him but it really seems that kids can just be very sensitive.
I think it is pre-puberty or emotions getting the best of them.
I also heard it was a full moon this week. lol