I had a dream last night that I was having a conversation with my boyfriends ex girlfriend and one of her friends. She was nice to me and we had a civil conversation, but in the dream I told her that my boyfriend was over her. Her friend replied, "no sweetheart. He's not over her and this isn't the end of these two." and his ex just shook her head yes.

What does that mean? Is that a sign that my boyfriend and his ex will get back together some day or he is talking to her behind my back?

This is what ur dream means

my guess would be that it means u 2 are not going to be together forever

I think it just means that you are worried that they may get back together. Are there any signs that they want to get back together. Does your boyfriend make you feel confident that he only wants to be with you now?