Dude I am 100% right there with you. My GF thinks I'm always cheating (which I never have) just because I work late one day, work on the weekend, park my car in a different spot than usual, workout more, etc. I'm exaggerating but I hear you. I've already stood my ground numerous times and have made it clear I will not put up with this BS of hers. However, it's a revolving door. It's a constant battle and frankly I've come to realize she likes when I put her in her place. She likes to be checked every now and then because she wants the confirmation that I am emotionally invested in the relationship. I've tried the "aloof" reaction and it made things worse. This may not be the case with your girl but just wanted to offer another perspective.