Cheating?

My SO & father of my child (I'm 27 weeks pregnant) cheated on me four years ago by having sex with a stranger at the time.  I recently found out when we were split up for a year he had sex with the same girl he cheated on me with.... I'm not over reacting am I? I'm hurt. Before we got back together we were honest about our dirty laundry & he told me about other girls but not her.