skin keeps changing when entering different worlds

What actually happened was...:

Hello,

I play minecraft and i play on version 1.7.9 on a server. When I go to play Minecraft and my skin is the right skin. But when I go to another world by doing /spawn function on the server I play, it changes to a different skin such as Steve skin. I hope you can fix this so my skin doesn't always keep changing.

What I expected to happen was...:
It would stay the skin I made it.

Steps to Reproduce:
1. Login into minecraft, go onto a server thats 1.7.9
2. go to a different world
3. check your skin to see if its the skin you were hoping to play with on minecraft 1.7.9 . Which in my case it's not. Never had this problem before though.