My phone was frying

I listening to music on youtube as I went to bed as I usually do and as usual I put my phone under my pillow and went to sleep.

When I woke up and this has never happened before, when I touched my phone, it was really hot as if I was touching something that had just come out of the oven, I was kinda shocked, scared and horrified. It was so hot, I couldn't pick it up or hold it more than a few seconds. It wasn't just overheated, it was FRYING like an oven.

Anyway I was tired at the time so I went back to sleep listening to the music, when I woke up and touched it, the battery had gone, it was still slightly hot but not as bad.

I'm really confused.