Thank you for writing to us.
Cold sores are caused by HSV1 or Herpes virus 1. Herpes is almost always a sexually transmitted infection. HSV 1 is a disease which can be spread by kissing. Hence, if you kissed anyone with exchange of saliva, you could have transmitted it to your GF.
If you had sex before your GF, you may have gotten an STI and transmitted it to her. Hope this helps you. All the best.
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