What was bothering Johnsy as she lay on her sick bed in "The Last Leaf"?

We learn from the doctor that Johnsy, lying sick with pneumonia, has only a one in ten chance of surviving. This is because she has decided she is going to die. As the doctor says,

Your little lady has made up her mind that she's not going to get well. Has she anything on her mind?

Sue finds out that Johnsy has on her mind the idea that once the last leaf falls off the ivy outside her window, she will die. She has been counting the leaves falling from the gnarled vine that grows on the brick wall opposite her. The vine is now down to five leaves.
When the vine gets down to one leaf, the desperate Sue goes to speak with the old painter Behrman. She persuades hims to paint a last leaf on the vine that is so realistic it fools Johnsy into thinking it is real. That way, because this leaf never falls, she has time to recover from her illness.