Why does my iPhone overheats on the upper left back of my phone?

My phone is overheating even on the lock screen, just after few minutes of using it makes it hot and displays the overheat warning. I tried doing a factory reset. the camera and flash works, the wifi on my phone is bugging i think it sometimes connect but sometimes wont. (ive just had screen replacement) it was not done by me. The wifi is totally fine checked it a couple times its just the overheat warning turns off the wifi and i was sometimes far from the router.