Welcome to the community!
Do you have your GPS, Wifi, and bluetooth all on at the same time? If you do, it can consume a lot of the CPU usage thus result in overheating.
You might also consider limiting your GPS location services. Many apps use GPS location services. Unless you're trying to use some sort of map, having High accuracy is not usually necessary. It'll just add to the battery drain.
Have you considered modifying your app sync settings? Some apps you might not need to sync on a regular basis.
Additionally, I understand you don't think you have apps running in the background, however, some apps can run silently without being listed in the Running Apps section. I personally use an app called Greenify to hibernate all running apps which I do not actually need running all of the time. I understand some people suggest not using certain apps which claim to save or improve battery, CPU or RAM as they can cause a lot of problems with overheating and interfere with other core services. Indeed, I found CleanMaster to take up large amount of resources. So far, I've been satisfied with Greenify (I do not have any affiliation with the app or its developers).
Hope this helps