@SarahJCC: Sorry to hear that you're having issues with your Galaxy S20 FE. Do you have Lock Screen Shortcuts set up for the Gallery and Messages app? To check, head to Settings > Lock Screen > Shortcuts, and look at the Left and Right Shortcut options. If the shortcuts are in place then please try disabling the feature via the switch at the top of the screen, or choose some different shortcuts, to see if the issue persists.
if the above isn't applicable, please try restarting your phone in Safe Mode, as this will disable all third party apps, and help to determine if the issue is stemming from an external source. if you notice an improvement, restart the phone as you would normally, and delete any apps you have installed on the device prior to the issue occurring. Sadly it isn't possible to tell which app is responsible, therefore please approach this using a process of elimination.
To access Safe Mode, press and hold the Power button until the 'Power off' prompt appears on the screen then release > Touch and hold Power off until the 'Safe mode' prompt appears then release > To confirm, tap Safe mode > With Safe Mode enabled, ensure Airplane Mode is turned off.
