Why is my iPhone 13 Pro heating up?

Every once in a while I notice that my iPhone 13 is running hot. Today I noticed that it felt hot in my rear pocket. I checked it with an infrared thermometer and it registered 108 deg. and it indicated that the battery was low and it wanted to go in low power mode. This has happened several times both with this phone and my previous XR. What could be causing it to run hot? I believe that it could be harmful to the battery and circuitry.

[Re-Titled by Moderator]