skin always having black lines (always having classic model)

So, I logged onto Minecraft today and I decided to go into another perspective, and I saw that I had black lines on my back and that I was using the classic model instead of the slim model that my skin is supposed to be. So, I decided to change it and relog, but that didnt work out. So I decided to restart the client, which didn't work either. And then I tried to repair the client, which also didn't work. And then resorted to restarting my computer, which also didn't work-- And now I'm stuck with this. Black lines, it's not a big deal, but it bothers me a lot.