So, now I know the reason for the donut... I foolishly tried to rotate my tires this Sunday— including the spare, as I can't afford to get the one tire replaced yet, let alone all four, which is what really needs to be done.
Anyway, last night and this morning I noticed that whenever I turned my once-again full size front tire to the left, there was a horrible scraping sound, as if the tire was wedged against some inner part of the wheel well— which, of course, it was. It was definitely off-centre in the wheel well, which probably means more money to fix it, but— ah well, it's only money.
So, this evening, before I came home from my first official day of freelance work (Woot!), I had to rotate the tires back so that the small wheel would once again have the place of dishonor, and would clear the well when I turned. Thankfully that worked, and I got home all in one piece. I think, though, tomorrow, I'm taking the Metro.