My boyfriend's ex contacted me saying they had sex!

what should I do? I asked him and he said no they didn't. He told me she started trouble and started harassing this girl he was seeing b4 me. Should I believe her? or is she making it up so we can break up?
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my boyfriends ex girlfriend contacted me saying they had sex.. while him and I are together***
My boyfriend's ex contacted me saying they had sex!
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