Can someone help me figure out how this happened?

I purchased 8 bitcoins today and had them sent to my bitcoin-qt wallet, I got an email saying they have been sent so open it up and see they have arrived but are unconfirmed, so I come back an hour or so later to find that they have already been sent from my address to another address that I do not own (afaik, unless I am missing something here) and my balance is now at 0 bitcoins.

How the hell does this happen? I expect there is no way to recover those coins now? It's at 3/6 confirmations now for this new address... Am I screwed? I'm pretty careful about downloading anything that could be malware or clicking links etc so I have no idea how this happened? Is there anything I can do now?