i am not entirely sure i agree with the other posts. just because ONE TIME when you fingered your girlfriend and she was loose, totally does not mean she is cheating on you. there could be any number of reasons she is "not interested in sex". try not initiating sex at all and see whether she starts something up. she may just feel not up to it, tired, stressed, feeling unattractive. could be anything. you could try talking to her about it, but just dont do it in an accusatory manner.