I see everyone's skin as a classic skin.

I thought it was just my skin where I saw black lines on the back and it was a classic skin. Ive selected slim skin and even on the launcher it says slim skin. But no matter what version I log onto, when I join a server, my skin is now a classic skin and I've tried to do everything to fix it. When I go to a sever, I realized everyone's skin is no longer a slim skin. They are all classic skins. Those skins who were slim skins, you can tell because they are now classic skins with black lines on them.