You know, there's a pretty big difference between a kiss and a full blown makeout session. If you respect your girlfriend the least you will do is cut off contact with the other girl outright. Whether you tell your girlfriend about it is up to you I feel as it is unlikely that she's going to contract some sort of disease from you via the original kiss.
The question is, did you kiss her or did she kiss you... Were you drunk?
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."