[emma01]

I think you need to just discuss this with her, bring up last nights dinner and politely ask her if she is eating enough. It may just be that she has a genuine lack of appetite, and if so, that should return when she is feeling a bit better!
If you feel she may be trying hard not to eat, try telling a parent or other trusted adult about her, it may also be good for her to see a counselor (if she isn't already) and you could encourage her to tell the counselor about her lack of eating.

It may not be an eating disorder yet, but it could certainly be developing into one, and if she starts liking this weight loss, you may be in for a long few years! Best to get it out in the open early on, so that she can get help!