Issue with username

I changed my name to "SavannahYaGurl" and so I go to play and I open up minecraft and it still says my old name which was "savannah08123" and so I keep restarting the game, it still says my old name. So, I go onto a server, and it says "Not athenicated with minecraft" on hypixel server, I tried several servers, it says the same thing. I tried single player and I typed "Hi" to see if my name was the new one, but it is still the old name that I don't want anymore, help please.