Iphone 13 hot when neither using or charging phone

As the title says, this morning I woke up and my iphone 13 pro was very hot to the touch. So hot that it almost hurt to touch the metal on the sides of the phone. I’ve never felt the phone get this hot before no matter how many apps are running, but this time the phone was locked and the only app being on in the background was the alarm clock app. The phone wasn’t charging and it was not out in the sun. The phone was under my mattress topper when sleeping. Could the placement of the phone have caused this? Or the fact that I don’t have the latest ios? Or strictly a hardware problem?