Is it a red flag that a girl I'm talking to lives with her best friend who's a guy and also has slept with this guy?

So this girl I've been talking to everything is becoming very serious were basically together but without labels yet early on she told me that she lives with her best friend who's a guy I didn't think much of it but lately I've been thinking more it she has slept with him multiple times but I asked her is there anything that I need to be worried about she said no she doesn't have feelings for him she has been spending so much time with me I believe her but something just doesn't feel right

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11 mo
I overreacted big time! Things have become so amazing between us! Everything actually feels perfect and I have full trust in her I believe her, she would never do anything that would hurt me nor will I ever. Never felt love like this.
Is it a red flag that a girl I'm talking to lives with her best friend who's a guy and also has slept with this guy?
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