why my iphone 5s ios 8.1.2 getting too hot?

my iphone 5s running through ios 8.1.2 suddenely gets too too hot and battery drains so fast, i downloaded an app called sys system to know cpu usage, i discovered that the mail (exchange & hotmail) are ruuning in background even if i closed the app , i tried to delete the two accounts but no solution , the only solution is to reboot then 1 or 2 hours suddenely getting hot again, please help