J.C.
I had something similar happen to me twice when I was little. Turned out that I was lactose intolerant. My doc at the time (was some time ago, lol) said that the only way to tell was to take me off of all dairy products for at least two weeks and see if that helped. I wasn't happy about it, but I did like not being in pain.
The second time it happened, it was anxiety. I loved my teacher, but there was one thing that she was very confrontational on. Imagine a small child being intimidated by their teacher! But it was only the one thing that she was like that, and otherwise I really liked her. Took forever for my parents to figure that one out, because they would ask if I liked my teacher, and yes I did.
I hope that you figure out what is going on with her, I can really sympathize with what she is going through.