I recently hung out with a bunch of girls and they asked me to go clubbing on Friday. Problem is, my gf of two years was there and she was 'claiming' me the whole time, being affectionate, kissing, etc.

GF works weekend and I'm completely free on Friday. I got a good response from one of the girls and I'm confident I can get laid that night. How do I go about explaining my gf though? Do I lie that it's an open relationship? Do I just tell her to keep quiet about it? Can I trust a random hookup?