Why does Catherine put herself into a fit?

She was srtuck by a fit in the midst of a quarrel. 

Mrs. Linton caps them all. I may say this; it commenced in a quarrel. She was struck during a tempest of passion with a kind of fit. That’s her account, at least: for she flew off in the height of it, and locked herself up. Afterwards, she refused to eat, and now she alternately raves and remains in a half dream; knowing those about her, but having her mind filled with all sorts of strange ideas and illusions.’