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.