when i change my name in minecraft and try to play on a server is says it is not "authenticated with minecraft"

 So Basically , I only use 1.12.2 or 1.12.1 and i wanted to change my name.. so i changed from SugarlySweetLoli To CreativeIsDed_ but when i pressed play it all looked fine it was working , but then when i wanted to play a server it didnt let me it said it is not "authenticated with minecraft" i dont know what to do .. so please help me