Blushing is usually an indication that someone is embarrassed.
Imagine a kid who quietly works hard on his/her studies and isn't all that social with the other students. One day the teacher has him or her stand up in front of the class and heaps a lot of praise on them for outstanding work. Essentially placing a "gold star" next to their name.
This lady may not be use to being appreciated by others and isn't comfortable or doesn't know how to accept compliments easily.
We live in an era where people oftentimes don't bother to acknowledge each other let alone use the words "Thank you".
Another possibility is she likes you and thought you might be flirting:)