I Have Very Vivid Dreams

I keep having very vivid dreams involving my ex and me and his new girl..
Last night I dreamed him and I were still together, and I was going through his phone that he got caught talking to her on. I put her on the block list and deleted her number out of his phone, and then I woke up.
But he was talking to her, and her name in his phone was something like "my babe" and mine was just my name.
Why do I keep having these dreams that involve information I find out in reality? This information shouldn't mean anything to me anymore since we're not together, yet it still hurts me because it's stuff that was going on while we were with one another and he acted innocent when I checked him on it.
Yet now his friend tells me that this whole time he was with me and I was confused on what I wanted he had already been trying to get this new girl. Having his friends hook him up!