Why does my phone heat up even when not plugged in to be charged?

HI Dottiej1, 

Thank you for participating in the Apple Support Communities.

Your device can heat up for many reasons, usually related to graphics intensive apps or games.

Please start with the info here: If your iPhone or iPad gets too hot or too cold - Apple Support

"You might notice that your device feels warmer when you:

• Set up your device the first time
• Restore from a backup
• Wirelessly charge your device
• Use graphics-intensive or processor-intensive apps, games, or features, including augmented-reality apps
• Stream high-quality video

These conditions are normal, and your device will return to a regular temperature when the process is complete or when you finish your activity. If your device doesn't display a temperature warning, you can keep using your device."

Are you seeing the temperature warning on your device? Are you in a warm area, warm car, or outside in the sun?

Have a great day!