might be running routine app backup

usually my lumia will automatic hot in

1. weak cellular signal strength
2. running backup
3. apps activity

if 1 then until the signal strength stable then only wont hot

common issue on weak signal strength place ..because phone will boost up it antenna gain to get better signal

my friend mia xperia also like that

@6216

925 got metal frame, so heating is more obvious. You could try hard reset if it's really bad.

Settings > About > Reset Phone

You need to back up your stuff and settings  as hard reset will wipe everything

Check the Battery Saver and see what is running.
Something is draining the battery fast that causing the heat generation. If you can't find anything, better send it for checking.
My 640XL do goes warm when charging at fast rate, but charging at slow rate nothing happens.

Looks like is better you send it for checks. OR try to reset it.

I see, still I feel something is not right. Don't you make it as the first Lumia that exploded. Samseng and Apple already got few cases

Well, hope this is the case. But mine are using minimum apps, so never face such problems

Basically I turned off everything. That's why 1 full charge I can use it for solid 7 days minimum
No money to pay mobile plan

Do you by any chance pin Cortana at your start screen? Cause its usually the culprit that makes the phone battery drain fast. Unpin Cortana and the problem should be solved.

So you are trying to say is some apps are fooling with you?

Well, do remember this : if a Microsoft product that does not hang or crashed, it is not Microsoft
Still, your problem are solved. Just do your self a routine, have a check on the Battery Saver to see what apps are running.