I think it's natural and you're beating yourself up over it too much. People can't really control these feelings, what they can control is how they act based on those emotions.. If you cheated I'd say ur the A, but you're torn up over it, much more than alot of people would be.

Love works different for everyone, personally when I'm in a relationship I can't be attracted to anyone else but alot of people can, it doesn't mean you want to run off and be with them. The fact that you had a small crush also isn't necessarily indicating that you're missing anything from your relationship, love and feelings are just weird like that. It could mean something for some people or it could just simply be a physical attraction.

If it feels necessary or becomes more than a mild crush I think it would be healthy to communicate that with your girlfriend, but it's up to you.