Samsung Ativ S overheating and battery drain

[WPCentral Question]

Hi guys, recently (after enrolling into the PfD and updating to 8.1) I started having problems with the battery on my Ativ S. Apps such as the Facebook Messenger, Internet Explorer or even turning the 3g on drain the battery severely, like 20% in 30 minutes. I tried HR a few times but with little success, the phone keeps overheating whenever I open the applications. Anyway the phone is rather old, because I bought it in December 2012 so I wonder whether it is the 8.1 causing the problem, or the battery is worn out itself and replacing it would help. Any ideas? Tia!