Boyfriend tries to set me up?

I've been with my boyfriend for 4 years. We went to a convention and day before he told me his female friend would be at the convention. He introduces but she is wearing this revealing cosplay her breasts spilling out and hugs my boyfriend. I give her the look and I asked him when we were leaving the convention how they became friends. He said he used to go on dates with her. I believe he lying to me why would he not tell me before the convention that he used to go on dates and he thought we would connect. Is it a red flag that he didn't tell me he used to date this girl and why is he friends with her?

Boyfriend tries to set me up?
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