Phone heats up
My phone feels hot since yesterday. The back of the phone is very warm
iPhone 8 Plus
It's perfectly normal for an iPhone or iPad to generate heat, but if a device always gets hot when used, it might be a sign of a hardware problem or issue with the iOS. More often than not, software is the cause of overheating issues.
See here why and when it warms up:--> Keeping iPhone, iPad, and iPod touch within acceptable operating temperatures - Apple Support
Start worrying if it presents a temperature warning message on your screen.
Hope it helps!