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Hi @Pretigalie,
Welcome to the community.
That's a little strange. Does it do the same thing in safe mode?
Follow the steps in this thread to get your phone in safe mode >>> http://talk.sonymobile.com/t5/FAQ/How-to-boot-your-phone-into-safe-mode/td-p/348008/jump-to/first-un...
Sometimes the act of putting it into safe mode is enough to fix the problem, so get out of safe mode by restarting your phone and test it, see if it happens again.
If it does, then the only way to determine what app is the issue, is to eliminate them one by one, uninstalling an app and testing after each one. You can usually narrow it down based on the issue. For example, in your situation I would look at apps that interact with the lock screen and the Camera.
Good luck @Pretigalie,
If you do identify the app in question, please let us know here, in case other users have a similar issue 