Phones heat up...
However, if you then see a rapid battery loss at the same time (above what's expected from heavy use) you may have another issue. Use
Battery to observe discharge rates when this happens and if it is above 20% per hour you might consider a warranty replacement if that is available to you...
That being said, there should be an automatic shutdown if your CPU core gets too hot and if that hasn't happened you have not reached the danger zone. I will say my 1520.3 gets hotter on occasion than most other phones I've used. It's kind of a known thing for the model.