Temperature warnings keeps showing up

Brief story about what happen, I was charging my iPhone 6s with my new portable charger I bought recently and I was face timing at the same time until the temperature warning showed up for the first time since I bought my iphone. So stopped using and let it charge through the portable charger.

After it was full charged, I took it off, didn’t use it for couple hours, and notice that the battery had drained to 49%. I continued using thought it was because of a app that I forgot to turn off. After 10-20 minutes of using, I noticed that the battery is draining fast and overheating until the temperature warning shows up again. Tried to use it multiple times and same thing keeps happening. Thus, tried to reset setting and turn off the power for an hr.

It did work a little bit but then it end over heating again with temperature warning. What do I do? Turn it off overnight? Battery replacement?