It’s a defense mechanism to reject someone you feel rejected by. But if you’re feeling rejected by the person you’re in a relationship with you need to resist that instinct and communicate how you’re feeling. You need to find out what you don’t know.
We tend to take everything personally when we’re falling for someone but it may not have anything to do with you. It could be work, family, or finances, all potentially stressful topics he may not be ready to share. You may be getting yourself all worked up over nothing. Make sure you’re not deciding for him what he’s feeling, because if you’re wrong (which is very possible) you will be the one to destroy your relationship unnecessarily. You need to find out what is actually happening and only respond to facts.
Don’t blame or accuse her of anything. Stay calm and empowered by using “I statements” to express your concern. Ask, “I feel like you’ve been a little distant is everything ok?” Give her the chance to calm your insecurities or potentially fix any problems with your relationship. If it does turn out to be that her feelings have changed you’re better off hearing it from her and ending it amicably. However it turns out you will feel much better by getting answers than making up all sorts of scenarios in your head.