Why is a charger icon flickering at the bottom of the screen?

It all started when my iPhone 5s was dead and said it was charging and finally turned on after 2 hours. There when I unlocked my phone, a flickering "frame" around the bottom of my phone. At the bottom, you could make out a charger icon, like the one you see when your phone needs to be charged. I did a hard reset and it is still there. How do I fix this?