Screen problem with macbook air

Hi! This black spot appeared on the side of my screen last night, out of the blue, and I'm not sure why or how this could have happened. I hadn't accidentally dropped my laptop or hit it against any hard surface, nor did I place anything heavy on my laptop. The size of the black spot appears to vary as well. When I turned on my laptop last night, it was about 3cm long. But when I opened it again this morning it was about 4cm long and slightly thicker (as seen in the left picture, though it's a bit difficult to see how different they are from this image). As I was typing this post the size seemed to shrink back to how I found it last night, so the image below shows the comparison. Can someone help explain to me what is happening and what can I do to fix this? So far I've only tried restarting my computer, nothing happened. I ran an apple diagnostics to see if it can detect any problem with the hardware, but nothing showed up.