Ender Dragon Spawned in Overworld when I went back into my saved world.

I spawned into my survival world two days ago.  I have previously defeated the ender dragon in the end world.  As I was walking a little farther from my base, I hear the sound of the ender dragon.  I climbed up the mountain and an ender dragon was flying around burning up the entire area.  The life bar also showed up on my screen for the ender dragon.  I exited the world and made a few saves to help save my world.