Why is my phone overheating when it's charging

If you use it while it's charging then I think that's why, but if not then I think it's a overheating problem or if you use a power bank to charge it if available use a outlet because power banks can get hot. If none of this works go to your service center

@wiserchabinga that is usually caused by a bad battery. You are not telling us if anything happened to the phone before this started. I suggest that you replace the battery the re-evaluate.

Should a new battery not work, check the charging board. Check the connector and flex cable for any tears etc. Get an ammeter and see what amperage your phone draws from the charger. If the amp is high it is most likely an issue with the charging IC on the main board.

Since ifixit does not yet have a specific guide for this, use something like this video Replacement batteries are readily available at many online stores.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has a device with the same issues as you do. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!

Pretty common, use the proper cable for the device, maintain a good battery. Unless its causing external issues, its not a big deal.