Automatically switched to Normal and fire paralyzed me

I was crafting sugar cane in The Nether when I suddenly went up in flames and died. I checked the difficulty and it said Normal,so I must have been set on fire by some monster(since I'd checked to make sure I wasn't near fire or lava) even though I had been playing on Peaceful last time I logged out and didn't change it after I logged back in. This is the second time the difficulty went up two levels on its own,although this was the first time I was set on fire and couldn't even run back into the Overworld and jump into some water. I was literally paralyzed.