I think that the fast was to completion in one sense only, in that her body needed more sustenance. There must be some toxicity left in the body for the condition to return. This is not unheard of and I think it is more common nowadays with the excess of processed rubbish we are ingesting which is more of a burden on the body to remove than the foods eaten in the times when the bulk of modern fasting knowledge and experience we rely on has come from.
Successive fasts should be easier in general, as it is more a case of cleaning out nooks and cranny's maybe, but the house still needs more cleaning. That is the simplest explanation.