If you have recently/ever changed your nickname for your username for your minecraft account, you need to LOG OUT and LOG BACK IN to the launcher (just restarting your PC is not enough) and your account will be synced with itself online. This worked for me, there are other network fixes like using a different DNS server that might help you but this was an instant solution for me as i had just changed my username and could not figure out what was wrong and was not mentioned online anywhere.