Dream of having sex

I keep dreaming about my ex who cheated on me, and the girl he cheated on me with while I was pregnant. It was over a course of time, but we have been broken up for a half a year now, and the dreams just started about a month ago. I thought I had gotten over it but I do not know why I keep dreaming about them. I see them having sex in my dream, but he also chases after me when I run away and when I wake up all i can do is cry.