My Phone starts overheating since the iOS 8.4 update

hey, uhmm i just wanted to ask if whether my problem is a software based or hardware.

A few weeks ago apple released the new iOS 8.4 and I updated my phone with it. It worked liked a charm that day. But after it, the phone started to get warm and sometimes getting hotter without any apps running. Even on standby it is constantly getting hotter and hotter. I did some simple troubleshoots like turning off Background APP refreshes, mails, Network reset and all those stuffs that I might find useful. I already restored my phone on factory reset many times hoping that it might solve the problem, but its still the same. Oh and btw, when the phone started to get warm the phone shows "NO SERVICE" and sometimes on "SEARCHING". Turned the Airplane mode on and off just to refresh, it still the same. Its been almost weeks since the update and my phone is completely useless even if i charge it it drains the battery instead of charging. I visited some Apple Authorized service and they told me that the battery might be the cause they told me it was "Consumed" but how come it is consumed if the battery can still charge? Only if i completely turned my phone off. So I think there might be some other reason why my phone is warming up even with jsut doing nothing. Please help

Iphone 4s 16GB