at first she is very kind

maybe she doesn't love you anymore, or maybe you do the mistake that hurt her. women have weakness when you say romantic things, may be you can try to say you are so beautiful, you are so kind, i love you, etc.if it's still fail. leave her, search for the another girl

Maybe you did something wrong to her that make her angry or she might have big problem that worries her that much. If that is the case, be with her and wait for her to talk about it. Sometimes girls are hard to understand its their nature. If I were you, ask her for a dinner and have a good talk after. Tell her what you feel about the things that sudden changes her and relationship. Ask for forgiveness if you've done something wrong and offer a solution to save your relationship. God bless...