Is my best friend my best friend?

So I recently found out that my "best friend" is talking to this guy I really like on snapchat, i saw that her name had the emoji next to it showing your number one best friend is their number one best friend etc.. I asked her why and she lied and said it was this guy she was talking to named Shane and she sent me a screenshot of their snap streak and I saw another person with a long snaps streak who was the guy I'm talking to and said they are just talking to keep their snaps freak going.. I find this weird.. She also dated my ex and lost her virginity to him awhile back and I was mad at her for a long time but it's hard to lose your best friend after you share everything with them..