ID's can't just be stolen like that. The first 5 letter/numbers of the code you use to go onto your blockland acount, are not random. So anyone could have the first 5 letter/numbers. The rest is basicly impossible to "hack" because the internet is constantly changing if you are on/hosting a server, so basicly, the only possible reason is if you told someone. Also, delete any files on your computer with your key in them, like a memory for yourself. If someone is screensharing, they/you could have opened it up. Besides that, i can't help you.