Please help me!! I'd like someone to please help me!!?

Okay, here's my predicament. I love my girlfriend very much. So much, that I'm going to ask her to marry me this year. She doesn't know when of course. She just knows that I am. Sometimes when I am talking to her, we usually text because she is horribly busy during the day so we just text, on occasion I've said something that ends up upsetting her. She'll start texting like she is upset with me. I'll ask her what's wrong and she'll say, "Nothing, or nothing's wrong." I'll say okay and continue to talk how I've been talking to her. I KNOW that she's upset because she'll start just saying "Yupp, or yes, or just simply okay." I don't say anything to her about it because then she really will be upset because I've said something about it. That kind of hurts my feelings when she does that. It just seems like she doesn't really care to talk to me. She just keeps saying single words. Nothing much at all.

What should I do? Should I leave it be when she does that, and wait it out until she's okay? I'm confused. She won't tell me what's wrong so that's out of the question. I don't know if something is really wrong, or if she is just acting that way for no reason.Please help....