Was my girlfriend Cheating.I dont know what to believe?

Well yesterday i came home to find my girlfriend in bed with my best friend and they were naked.They told me they were playing leap frog and took all their clothes off because it was very hot in the house.She told me the white stuff on the sheets is from when she spilled some milk but i just don’t know what to believe.Was she cheating or were they just playin leap frog?

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