Yes. This is session hijacking. Where he gets gets your session key to go on your session and kicking you out.
The things that you should really do is kick him out, log out and change the password.
f*** I don't think i'll be wasting my time on this game yet. Will wait for Blizzard to close this loophole.

They're trying to get the best PR to handle this mess.
- Error 37.
- Multitude amount of server going down.
- Must be online to play a single-player game.
- Botched launching.
- And now, hacking. A hack that you can do nothing about.

Guess what? The game is just 1 week old... lol.
To Blizzard;