Why is my iPhone is overheating?

So yesterday i opened my system to see what was going on because my front facing camera was not working i found out that the flex cable was broken so i know i have to replace it during all of that i did not remember if i disconnected the batterie or not i am pretty sure i did well the thing is after that i realized that the phone was heating up i thought it was normal since it had 20% batterie life. so i went to charge it and is was hot i mean stove hot i opened it again and saw that the flex cable was very hot and bent all the way to the bottom i have not used my device since