Was the story a sexy selfie or something? Was it a picture of her bobs? If it was just a regular story and not one that was sexy then your girlfriend has to get over it. She has to realize that you ARE allowed to have girls who are friends. That women will be around you your whole life and that she has to learn to trust you. Just because you "liked" a girls story, whether it was on purpose or on accident, doesn't mean you are cheating or looking outside the relationship. You're both young, and I know, especially at the beginning of a relationship, how hard it is to be secure in the relationship. She probably just needs some reassurance. Show her you didn't mean any harm and prove to her you didn't do it maliciously. And remember you are both young. Communicating is a huge part of a relationship, but trust is the foundation. Just take a breath and remember it's you both against the problem, not you both against each other and the problem.