Hi Griff,
congrats on getting "cave girled".
I partially agree with Prince, if there are no witnesses keeping your mouth shut it the safe way to go.
But somewhere in the future you or a witness can get drunk or emotional and start talking... and these things can blow up in your face. Not because of what you did, but because you diddn't say anything. The whole thing will become a trust issue etc etc...do the math...
If you do decide to tell her (I would), make it perfectly clear that it happend in a flash, you totally resented it and that you did everything in your power to stop it. Hell, it even made you feel miserable and miss her! She has to understand that you can't just bite someones tongue off right?
I think the odds are in your favour, perhaps a little fight and then some makeup sex

haha.
But I'm not sure what your relationship is like and how sensitive your gf is... so no garatuees...
Perhaps some views from the other guys on this?
grtz and good luck!
Bucc