Most probably causes are the running of too much background apps. games, video etc.
Try disabling or uninstall some junks apps in your phone. they are quite alots. It have lovely name but even now I dun know it really work or just conning you.
Ok. I give u an example u can test yourself.
On my MI4 I have a garbage cleaner call Clean Master. Yeah I know u all will swear that it work. Me too at first. ok i started it up and it cleaned 605MB wow!!
But I still Have another cleaner call Power Clean and it nice too. so I fired it up and and do the junk clean.
And It find 509MB in which I just clean 605MB with clean master. So I clean the 509MB which Cleaner did not find. Good!!! Now I start my clean master again and see if there are any more junks to clean. and it Find another 402MB junks to clean
To Cut the long stories short. u can go thro. this endless loop and there is always something to clean. But if u use either the SAME program twice. it will say there is nothing to clean