When I was on the inernet and playing youtube videos, I noticed a line came on my screen than went away very fast. It wasn't in the internet window. Another time in youtube a quick line went across the middle of the internet window and went away. Then just a few minutes ago, I closed an internet window when I was watching youtube videos a quick flash came on the corner of the screen where I closed the window. Should I be worried? I noticed that the back of my macbook was pretty warm when I closed the lid.