iPhone Gets VERY Hot

I just got the iPhone SE. It's very nice but I'm concerned about how HOT it gets. (This is what happened with my 5s when the screen started to split. The phone worked great until earlier this month when I noticed how HOT it would get whenever I used it.) It did NOT have this problem until recently. A few people said it was the battery.

I had the 5s for a while and decided to upgrade to the SE. But now I'm worried that the same thing will happen to my new iPhone! I've heard some real "horror stories" about Apple products but this is the first time anything weird has happened to ME. What did I do wrong? 😢

Both phones were purchased at T-Mobile. I still have the 5s because it wasn't eligible for a trade in with visible damage. The SE is brand new and still has a one year warranty. No Apple Care coverage. (Can I even get Apple Care if the phone was purchased at T-Mobile??)