It's possible but I wouldn't jump to that conclusion. There are a lot of conditions that take away the appetite other than eating disorders.
Also, not all people eat on the same schedule. I have family members tell me that I never eat, but that's not true at all! I just eat at different times.
If she does show signs of an eating disorder, I would talk to *her* about it. I wouldn't go straight to an adult without consulting her. You don't know for sure that there's a problem just because it seems like there is one, and if there is one, you don't know that that's what it is. I can tell you that when I was a teen, I had people go straight to adults about things they thought were a problem in me and it created a lot of problems and didn't fix anything. If she has a problem, she needs to be given the opportunity to be the one to deal with it in what way she feels best before someone else intervenes. It might come to that, but you're not there yet.