When I was too thin (out of college, at 105lb), the way I could tell I was too thin was that I got dizzy and lightheaded a lot. The way it got fixed was Spouse moved in and made sure I didn't skip dinners in addition to skipping lunches, and my stress mode of not bothering to eat didn't kick in as strongly, so I'm not sure that's any help to you, but my point is, so long as your body is not feeling unwell and is not picking up new odd reactions, I'd not worry about too much about what it weighs.

If you're on an appetite suppressant, though, you might not be eating enough - so if a craving manages to hit you anyway, do your best to indulge it.