If you get a vaginal bacterial infection does this mean your partner is cheating?

Certainly the situation merits a frank discussion with the partner. If you've had genital-genital contact with someone else during that time, it's possible you brought the infection into the relationship without having sex with someone else. It's possible your test was a false chlamydia test result. A more obvious and common explanation is that your partner brought the infection into the relationship. Sorry for your trouble, whatever the cause may have been.