Girl who used to F your man!

I work at a beauty school and there is a student here who used to hook up with my fiancé (way before we were together. We've been together 3 years now) and I feel uncomfortable...but am I overreacting? Obviously this happens in life and I know I'm not the only person he's been with but whenever I see her all I picture is my fiancé f-cking her! UGH.