Battery problems
MY iphone keeps on getting VERY hot then I get then temperature warnings and my battery goes up and down, for ex: it'll be on 60 then it'll go down to 10
iPhone 6 Plus, iOS 8.3
try to turn off icloud and photo share on your phone. And any background working app. If that doesnt work then u probably have a virus by downloading whatsapp videos or photos sent to u , or u sync smth with virus to your phone. if so, then restore your phone after u back up your data