I do not think that someone is using your account, as this problem happens to a lot of users when their connection isn't optimal. Just restart your Minecraft and you should be fine. If the problem still occurs after this, then please check your internet connection.
If these two options do not work, and you are still getting that message while joining, then you should contact a staff member through Discord. They will be able to kick the account, which will kill the connection. They will also give you support if your account indeed is hacked.
Thread is now locked, as multiple solutions to this problem have been given.