The thyroid test will tell you for sure. But being thin and having an irregular heart beat can both be signs of hyperthyroidism. However, that's fairly common and fixable so it's not something to worry much about. I am pretty much the same size as your daughter and I'm 34. I eat plenty, some people are just naturally thin. The key is to make sure she's eating through the whole day. I have a lot of small snacks like nuts, yogurt between meals. I would be a little concerned about all the soda, that could be filling her up and making her not eat as much as she should. Especially if it's caffeinated soda. I know when I have caffeine it suppresses my appetite and I can skip a meal without even noticing.