In Howl's Moving Castle (the book), what happened between Calcifer and Howl when Sophie's heart played up?

It says in the end of the book that Howl and Calcifer had known almost the entire time that Sophie was with them that she was under a curse so, yes, this is an instance of Howl and Calcifer working together to try and remove the curse from her. I think what it means about her heart is they were trying to get it to beat like it should, as if she were a young girl, but it went back to beating like an old woman's.