"You're not my friend" is not really bullying. Went through a lot of it with DD1 in Prep. It is just them learning about play and friendships. Often when they say that it means "I don't want to play with you right now" or "I don't want to play the game you want to play". As time goes on they will learn about being friendly, what friendship is and how to compromise game play
Often DD1 would ask to play, which gave the other children a chance to say 'no' rather than just joining in. Sometimes they just like to say No just because they can.

DD2 is tired. The hot weekend really killed any rest she got in the lead up. So she is pretty cranky and did not really enjoy her first gymnastics session this afternoon.