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.