black smudge in the upper left corner of the screen

I haveMy screen isn't broken nor does it have scratches but a few days ago a black mark appeared on the top left corner of the screen (for seemingly no reason). I updated iOS in hopes that it'd help but it looks like it got worse instead. It also looks like it's getting longer towards the bottom. The apple services where I live are pretty horrible and I've heard lots of stories about how phones that were sent in with warranty were made worse instead. So I'm a bit hesitant to just ship it to a service. What do you think this black mark might be? What's causing it? Lastly, would it be covered under warranty since I didn't really cause it to happen myself?