Boyfriend dated my friend

So I just found out that the guy that I've been seeing dated one of my friends. It was like 7 years ago and they only dated for a few months but they did have sex. They ended on really good terms. They're still close friends now  but even though we are completely different and he doesn't know that I know about them I feel like I'm being compared to her. It's just a really strange feeling that he has seen me and my best friend naked. How would you feel? How do I stop myself feeling compared to her? She doesn't know we are dating btw