2g gets very hot when charging

[Toymaker]

Last night I woke up and grabbed my iphone to check the time and it was very hot. It was plugged into a charger, so I unplugged it right away. Thankfully it cooled off after that. I'm nervous to charge it again, worried that it could be a fire hazard. Has anybody heard of this happening before? Do you have any suggestions on what could cause it or how to fix it?