iphone boiling hot?

basically at night i close all my recent tabs, put my phone on silent and put it under my pillow to go to sleep. this morning i woke up to it completely out of charge even though it was almost full last night and it was really really hot to the point i had to go cool it off by putting it under some cold water. any help? or explanation as to why this happened? i have an iphone 7 and i got it brand new in january.