Another question about a passage

In chapter 49 on page 404 pip walks back into Miss Havishams house peeks through the door, and all of a sudden she jumps up all on fire and runs at him! there are a lot of things i don't get about this scene, for one, how does that effect the plot? why would that add anything to the plot, especially since she's all OK and everything, and secondly, how did she even catch fire? was she just too close to the hearth?