Question: My iPhone is always heated up even if I'm not using it. When I charge it, it heats up even more and battery drains very quickly. Its battery is currently fully drained for more than half an hour and the back still seems as it is heated. What's the problem and how can I solve it?

Answer: 
Are you using IOS 7? When using IOS 7 be sure to check your apps which are running in the background, but normally these apps are also closed after some time in standby. Be sure to update your iPhone to IOS 7.1. When you say your iPhone/battery heats up when loading there is surely something wrong with your battery. Your USB transformer might heat up a little, but never had an iPhone heating up when charging. Contact Apple Support, or your reseller. Probably your battery needs replacement.
It seem that you battery has reach its limits needs a replacement. You can check to see if there are apps causing your battery to drain quickly. If this is the case you need to update your app. For instruction on how to do this visit <http://www.technobezz.com/iphone-battery-life-drain-dies-fast/>

I hope this helps.