Your girlfriend deserves to know.
You knew what you were doing, so I don't understand the 'somehow it led to sex part'.
You do not love your girlfriend as much as you think.
Tell her as soon as possible, and offer to drop out of the Nice trip and pay for a change of name to one of her mates or family members.
You had the guts to sleep with another woman behind your girl's back, have the guts to tell her.