If it getting really hot then there is a problem. However, knowing for certain what the problem is would be impossible to say. That said, the usual culprit is a battery.
Before doing anything else with the phone first... Is the phone is still under warranty? If it is, then you should be contacting your service provider or the manufacturer to see if they have a fix for this, will fix the phone or even replace it.
If the phone isn't under warranty any longer, then I would suggest you get a new battery and see if this resolves the problem.