If in doubt, get her chcked out, this is a baby we are talking about, they cannot tell you what is wrong, it could be minor, but it could be something serious, just because a baby does not act sick does not mean she is not. Please do not get me wrong, I am not jumping you, I am no Dr. It could be that mothers milk is not enough, or it could be missing something in it , you might need to use a supplement for a while or medication. At least call the dr on call and see what he suggeset untill you can get in and see your regular doc or baby doc.
Good luck and keep us posted on what you find out.