Jodi,

How many bottles of formula is she drinking? Formula is heavily fortified with stacks of iron, so I would pretty much bet Nina is getting enough iron from formula.

Has Nina started doing anything else new? Crawling perhaps? I always notice that when they start doing a new thing (crawling, walking, talking etc) that they get absolutely knackered for a few weeks.........(Lexie has just started crawling and she is, as such, off her food and SO tired.......). But then they 'catch up to themselves' and revert to being more energetic again once their little bodies have got used to the new activity?

That may not help..........not sure.