Remove any iPhone protector cover and then check again.

It is not clear here in your post about the ambient temperature in your area where you are using the iPhone. However --> Keeping iPhone, iPad, and iPod touch within acceptable ...
Store the device where the temperature is between -20º and 45º C (-4º to 113º F). Don’t leave the device in your car, because temperatures in parked cars can exceed this range.
Use iOS devices where the ambient temperature is between 0º and 35º C (32º to 95º F). Low- or high-temperature conditions might cause the device to change its behaviour to regulate its temperature. Using an iOS device in very cold conditions outside of its operating range might temporarily shorten battery life and could cause the device to turn off. Battery life will return to normal when you bring the device back to higher ambient temperatures. Using an iOS device in very hot conditions can permanently shorten battery life.

If you see a temperature warning screen
An iPhone showing this message may still be able to make emergency calls. To resume use of your device as quickly as possible, turn it off, move it to a cooler environment (away from direct sunlight) and allow it to cool down.
If your iPhone, iPad or iPod touch gets too hot or too cold

To identify counterfeit or uncertified cables and accessories, look carefully at the accessory's packaging and at the accessory itself.
Identify counterfeit or uncertified Lightning connector ...