Chastity is a thing even with non virgins. There has to be other concerning behaviors for someone to conclude she is cheating. She might just be servicing herself, imagination is enough to prevent horning around. She might just be so ready to get married she wants to hurry things along by depriving you of sex. See if you snap or show any red flags. If she's really doing the chastity thing that would be more of a good weird than a bad but definitely a smidge odd