How old are you? How old is your GF? You should consider adding age as part of your story as it helps readers provide appropriate advice.

Stop being angry, that won't help anyone. Have a real talk with your GF. Express that even though it may have meant nothing to her, it means something to you.

I've [32F] kissed plenty of women, including close friends, but am strictly a heterosexual. However, I have never kissed one whilst in a committed relationship, unless it was during a consensual shared activity.

Express how it made you feel in a mature way and I'm sure you two will come to a mutual understanding of the event and everything will be ok.