It's a compression artifact. (What @endpin said.)
You need to render at the initial shooting resolution, which you might experiment with, like full HD (1920x1080) and only 24fps.
And wear a simple, dark, solid color shirt so you're not making the compressor go mad with that, too
.
P.S. YouTube is going to compress the snot out of what you upload *even more*, so send them the best, most original shooting format.