Hey, if you're on premium minecraft, that shouldnt be happening, so re-install your launcher, if your on cracked minecraft, welp usually i have the answer but its hard to explain this skin thing, soo why dont you go here-
https://tlauncher.org/en/ sign up or sign in with the username your mc is and then go to your user area by clicking your username and then, you can either upload a skin or go to the catalog and choose one
Hope this helped,
Thank you,
And have a great day! :)