That's standard teenage girl behaviour I'm afraid. Sandwiches are right out in case they don't meet the correct social criteria - which is impossible, becuase it changes daily, and school dinners are far too naff, so the 'cool' in the school skip lunch.
As long as your daughter appears healthy, don't worry about it. Teenagers are physically far more resiliant than we as parents imagine, or indeed remember. It's one less battleground to war in, and let's face it, we can all do with minimising reasons to fall out!