Hi jesscortess16,
That certainly is strange. It sounds like it could either be a software glitch or the display itself is artifacting. I would attempt the following:
1) Force restart the phone by pressing and holding power & volume down (for iPhone 7 & 7 Plus), power & home button (for all models earlier than iPhone 7 and 7 Plus) or quick press volume up, quick press volume down and then hold the power button (for iPhone 8 and 8 Plus).
If this doesn't work
2) Make sure your iOS is updated to the latest version.
If this doesn't work
3) Contact Apple support and see if you would need to schedule a repair.
There may be other advice that can be offered that I have missed out, so wait to see what others reply to this problem with too.
Hope this helps.