I have SAMSUNG GALAXY S3 and i updated it from jelly bean to 5.1.1 lollypop and have one big problem

[AC Question]

I have SAMSUNG GALAXY S3 and i updated it from jelly bean to 5.1.1 lollypop and have one big problem. After like 10 ar 5 not working phone minutes , my screen just like turn off.. i press every key to on it , but one way it's just take battery off ( out ) . Maybe it's phone , or settings problem? My LED is working , while my screen just turned off..

 

[belodion]

A failing battery perhaps?
An unstable ROM?