installed apps recently? especially from sources other than play store?
I had installed apps in the past and my phone would get hot just being in my pocket. uninstall the culprit app and everything was fine. have had to factory reset before.
I left my phone to charge to full after restarting, it's only gone down 3% in the last half an hour or so. Checked the battery usage, only screen and game usage there now. That was kinda scary, my phone was hot after not using it at all. I'll update if something weird happens again.