It might not even be a cold sore? it came on friday last week and its already healed up? seems rather quick especially if it was initial outbreak which tend to be more sever, or so i hear!
Sure, it's a possibility. You could have gotten it from being unfaithful, or you could have had it for years and years, and gotten it as a child from an adult who kissed you.
She could have had it for years and years, too, and entered your relationship with it.
There's no way you'll ever know. It's just a cold sore. Don't freak out over it.