After playing a playlist, my iPhone automatically plays all of my music on shuffle ios 8.4

I usually listen to a playlist to help me get to sleep; it's about 7 songs long and I always shuffle it from the top of the playlist tab. However, when I wake up 8ish hours later I find that it's playing songs that aren't even in the playlist halfway down my music library. It seems to have added my entire music library to up next, and I don't know how this could have occurred. It's a drain on my phone's battery life as well as my sanity. There's probably a really obvious solution that I'm missing, but I can't for the life of me figure it out. What is wrong and how do I fix it?