I guess that depends on your definition of overheating. Is it warm to the touch or hot or too hot to handle? As a reference point, a hot coffee will be ~60-70C. Is your phone that hot or hotter? That would not be normal.
As Ben says, it could be some background apps running. To test it, close all your apps before you charge and see.
That said, when a battery is discharged, a charging block can deliver 1A of current to the device so it will indeed be warm. Are you using an Apple charging brick (the small square one)? If you are using an iPad brick or a cheap dollar store one, you could be driving too much current. You want to avoid the cheapie ones as they can damage your charging circuit on your phone.
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