Could be lots of reasons,
Do you think you may have done anything that's upset/annoyed her? Girls are weird, it doesn't have to be a major thing it could be something quite subtle (i'm guilty of this sadly).
Or she may be busy/very distracted when you're messaging, we all go through periods like that.
You may just be reading into it too much, I say calm down a bit with the messaging and see how things go. Then if she stops messaging completely then ask her what's wrong.