It's not normal for a nursing mom to carry a baby for 3 days. There are two causes for this problem that we typically see. The first is discussed in this thread and deals with a female who has become a child as a result of an Island Event. The second cause is described in this answer in our Support FAQ and deals with inadvertent or intentional adjustment of the computer's system clock.

As far as her health is concerned, if she is sick, you should have another villager heal her. If she is only weakened, she should recover her strength over time if you maintain an adequate food supply for your villagers,