Anxiety

Why would a 4 year old have anxiety about getting sick? Have we talked to much about the fact she was very sick when she was born? Tonight my niece was asleep when her mommy said, "daddy, would you take her up to bed now" and my niece started crying and said, "but I have to take my medicine." We reassured her that she wasn't sick and would be okay but her little tiny legs just shook. Seeing her this way just hurt my heart. How would you handle this?