Overheating after battery replaced

My iPhone 6s Plus needed a new battery. Took it to our local batteries plus store. We do not have an Apple store here. Since the replacement My phone keeps getting a message that it is overheating. It doesn’t feel all that hot at that point. But I shut down and let it cool. Batteries plus replaced the battery a second time. It’s still over heating. What could be wrong. I use the charging cord that came with the phone when new. My phone Is about 6 years old. I really do not want a New 5 g phone. Thanks for any help.