Ok, all you can do is go no contact not only for her to process and for you to reflect. While this woman who contacted your gf is a bit unhinged, your gf was certainly not thrilled about "drinking and staying in a hotel room" with other women.
Particularly that she heard it from a stranger who could easily verify the date, time, place, your name, your friends, etc which ironically made this woman more credible than you, since you of course never mentioned this incident to your gf, hoping it would blow over undetected..
We got chatting to a bunch of girls. we went back to their hotel to keep drinking. I fell asleep and woke up in a panic.
This girl new my name and searche for me on Facebook. Then the message was sent to my girl friend.