Why does it bother me that my girlfriend had one one night stand before she was with me?

Weve been together over a year. She is a great girl, never cheated and I totally trust her. I'm only the 5th guy she's been with and she has only had sex twice outside of long term relationships. Obviously the one night stand is one of those. All is great and never have I suspected her of cheating but for some reason just recently this has begun to bother me. She isn't slutty by any means but that one night stand just puts doubts in my head for some reason. Do I just need to get over it or what?
Why does it bother me that my girlfriend had one one night stand before she was with me?
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