Nexus S and phone charger get unusually hot when plugged in

I was using my Nexus S today and it turned off suddenly. I plugged it in to charge and saw it load to the boot splash screen, and then I went to sleep. When I woke up 2 hours later my phone was dead. I plugged it in to the charge for another half an hour, but both the phone and the charger started getting very hot. I tested my charger and battery with another phone and both seemed to work properly without the excess heat. What happened to my phone that's causing it to heat up like this?