Why my phone overheating

Enable developer options on your phone by clicking the model build number under the list of info about your phone multiple times. With developer options unhidden, find a option called: "running services".

This will show you every program that is running on your phone, there is likely one or more that you didn't know was active before. Its up to you if you choose to disable background process for the troublesome apps you find. Note, this can also help lengthen your battery life.