Is there any way to see how did I get it and when does it expire? I remember having a 1-week GA medal in my inventory, but AFAIK I didn't use it yet.

check your dashboard what probably happened is someone gifted you a buddy GA

If it was a buddy GA, there should be an event under the event tab which tells you which user gifted it. I have, however, seen a bug report that says the event doesn't always show up.

Personally, whenever I've been gifted with a buddy GA the event was there.

Well I said I was here before, so um yeah I to be the name changer all the time (AKA Soundbyta/Delakitty)