The answer is that yes, something seems to be going on there, if it rises to the level of "Anorexia' is unclear. The way you tell isn't just by asking (most deny it), but more importantly, by her behavior. And with
ED's, it an easy thing to monitor: She either gains weight, stays the same, or loses. The task here is not to just "Watch it", if it's there it will be very apparent very shortly...but to have a plan to address the findings.
What I typically do is meet with everyone and explain to the kid what the concerns are, and then establish a baseline weight she needs to be (usually, not a further loss), and if a certain time period, say, two weeks, if that changes, it means she has a problem which needs to be more directly addressed.
The pediatrician would do well to set you guys up with a professional with some experience assessing and treating these matters.