Normally I WOULD say that it is the actual display (the screen BEHIND the glass), since Samsungs are notorious for doing that. However, you have had it working for 3 days so... I've got nothing. Sorry for my uselessness X-P

This can be easy to fix if you haven't already. You will lose all of your local data, though. If it ever says that, press Refresh+Power and it will reset it. If you still see this, press CTRL+D to "enter" developer mode. When it has a RED exclamation point (not a YELLOW) press space to re-enable OS verification. It will wipe local data, reset your operating system and you should be good to go.