Phone overheating (over 40°c)

My phone starts overheating and the battery temperature goes over 40° mostly when I'm using 4G data with my wifi hotspot on.
I don't know why this is happening , I've looked all over the internet. If I try to plug it in , it will show its fast charging but instead the charge would start depleting.

from Android Authority Forums http://ift.tt/2bXUGJR
via IFTTT