The last person to use your key I would assume. All I can think of is a brother/sister/relative using your computer and changing your name to play on your blockland OR someone else has access to your key.Also ambiguous topic name, please avoid in future.
That name change happened from the first IP to use your key and it's from the same ISP that you use, so I'm assuming someone in your house played Blockland while you weren't looking. Also, in the future do not post ambiguous topics.
Where did you get this key? Did you buy it yourself or did someone give it to you?
According to RTB's ID roster, 'cheezy1254' is the name used by ID 16986, and 'danny135' is the name used by ID 16988.
I bought it myself.