Heard a twang sound on my ride to office, stopped and looked around. Didn't see anything, but decided that it was a piece of debris flying off. Also noticed that tire pressure was quite low. Unfortunately, I attributed my slow riding to the tire pressure, and reached office (about 1 mile from the twang) before realizing that it was getting harder to cycle. Turns out my rear wheel is severely out of true now, and rubbing against the brakes (and also the fender!)
I'm feeling pretty stupid right now, and annoyed at myself for not noticing this earlier and getting it fixed. Will be taking a shuttle from my office to a train station near my apartment, will walk it from there. Grr.