Apple Jack was hard working, and she didn't stop for any flu. But this.. The heat makes her feel like she's in lava, but she must prevail.. Right?

Basically, Apple Jack (And other ponies) get an influx of magic. A dangerous amount. And it turns into a small amount of heat to try to slowly get out of the bodies without causing any excessive damage that would happen if it just spilled all out. However, it still isn't a good time.