Yeast infections are not normally spread by sex (they can be) but usually yeast infections in women occur because of abnormal ph levels in the vagina which causes bad bacteria to grow.
However bacterial vaginosis (BV) is spread by sex and is usually the #1 reason for contracting BV.
The doctor won't be able to tell your girlfriend if in fact the infection came via you having sex with another girl - the doctor could imply that it was sexually contracted. But could not prove that it was from you.
Perhaps if you are having sex with multiple partners you should do so as safe as you can?? At least then you might be able to avoid such conflicts.