the doc can diagnose your gf with a yeast infection, but cannot determine whether or not you gave it to her. she could very well have developed a yeast infection all on her own as many thousands of women do all the time. while you could have passed it to her if you had had sex with a woman who had a yeast infection and then had sex with her, there is simply no way of knowing for sure how she got it. yeast infections are not STD's even though they can be transmitted via sex.

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.

That is actually usually Bacteria Vaginosis, not yeast infections. I'm not saying it's impossible :o} but more likely to be BV.