My iphone restarts on its own while I'm on facetime

I updated to iOS 7.1. Ever since I did, everytime I prolong staying in facetime audio for at least 40 mins, my iphone restarts on its own. One time though, it completely shut off despite how many times I tried turning it on, but it did work when I pressed & held both lock button and the home button. So my question is, why is this happening? This never happened on the previous iOS.