Try restarting the phone (turn it off and back on) this should stop the problem. If it doesn't, try starting it up in "Safe Mode" (instructions below) and see if it happens in there. If it doesn't, then you need to find what app, virus, malware, etc., on the phone is the cause and then remove it.
To get into "Safe Mode" please do the following.
1. First turn off the phone.
2. Next press and hold "Power" until you see the manufacturers logo on the screen, then release the power button.
3. "Quickly" press "Volume Down" and hold it for few seconds until you see the message "Safe mode: ON" or something similar.
4. If you wanted disable it, simply repeat the steps above, and you should see "Safe Mode: OFF" rather than on.