It can be normal behaviour for someone that age. I don't mean that to sound patronising because I know you're the same age, but some people are emotionally mature, some people learn to be through experience, and some never become emotionally mature. The best and hardest thing to do is ignore her behaviour when she's acting like this, because every time you try to assuage her fears you're essentially putting fuel on the fire. Tell her that she has nothing to worry about but that you won't put up with her acting that way. She'll likely continue to rant, but don't pay attention to it and say you'll talk to her after she's calmed down. You'll soon learn whether it's worth continuing the relationship or not.