A co student has suddenly turned from amiable to rude?

There is a student in my class whose recent behavior I don't understand, granted, she is not a personal friend, but we were cordial to each other now suddenly when I say hello, she has actually gone so far as to turn her back and ignore me. I tried to approach her a couple of times to ask if I had said or done something to offend her and I was met with the same rude reaction, I don't get it, (I'm 49 going back to school, she seems to be in her late 20's) yesterday, I completely ignored her (give her a taste of her own medicine) but to be honest with you I don't like to conduct myself that way, I'll be stuck in class with this young lady for one more month, just passing someone and not speaking to them is uncomfortable to me, what should I do?