I dont know what to do here?

So me and my long distance boyfriend have been together for 3 years. I know its a crazy amount of time but due to visa delays and the pandemic this is the situation we are in. Me and my boyfriend are 28. A girl on instagram who is only 21 put a photo of her and my boyfriend on her story and followed me. I texted my boyfriend to see what was up and he said that they were just friends and nothing else but that they had hung out together a few times as friends. Which im obvs not ok with but she said they had sex and that she is in love with my boyfriend.. My boyfriends strongly denies this. But she also said tjst she went out with my boyfriend and his brother. His brother said that this was all lies. So i know she lied about this. So I dont know what to believe anymore. I normally trust my boyf and he strongly denies having sex with her.
I dont know what to do here?
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