Isabella changed from a fine lady to a pale one.Explain

Before she marries Heathcliff, she is a rather shallow-minded young lady, pretty and quick-witted but a little foolish (as can be seen by her choice of husbands). Her unhappy marriage brings out an element of cruelty in her character: when her husband treats her brutally, she rapidly grows to hate him with all her heart. These very negative emotions take their toll Isabella and she becomes sickly and pale.