It sounds like your relationship is still pretty new. I believe you said you've only been together a few months? My fiance and I, when we started dating, waited four months before we even started saying it. And I believe it was a couple months after that before we got used to saying it all the time.

Has your girlfriend dated a lot in the past? Has she had many serious relationships? Has she been intimate with others before you? Maybe she isn't used to serious relationships yet, or she likes to have a sense of independence while dating (thus not much PDA). Perhaps she second guesses herself or is shy. Maybe she doesn't like to rush things. There could be a million reasons for her behavior. I feel you should talk to her about what has been on your mind. Try to be gentle, otherwise she might feel the need to be defensive. I think the key to relationships is honesty and communication.

Try to remember that just because you are in a relationship, you are not entitled to sex. Being intimate is an important part of a relationship, but both parties have to be comfortable, respected and feel safe.

Take care! I hope you are able to talk to her and work things out.