Well, that was fun while it lasted, but the Great Boob Crisis of February seems to be over. Whew! I don't know what it was, but after much massaging, heating, and (unbelievably painful) breastfeeding, the lump is gone, and most of the soreness has dissipated.
The lovely M-B asked how I got my supply up on the left. I'm not really sure what actually worked, but what I did was:
a) ALWAYS fed Eggbert on the left first. Sometimes I just let her finish on the left (she was usually happy that way, which is why I was always suspicious that it was a pumping problem, rather than just a supply problem), and sometimes I switched her in mid-feed.
b) When pumping, if I needed e.g. five oz, I didn't just pump until I had 5 oz (which invariably resulted in 4 from the right and 1 from the left), but rather pumped until I had half from the right (or a little over half, in the beginning), and then stopped pumping on the right and just continued on the left until I got the other half. My left is MUCH slower in production, so pumping for the same time on both sides meant that my right was doing all of the work. If I didn't get enough from the left in a reasonable period of time, I stopped, but then pumped again in a half-hour or so until I had enough.
c) Breast compressions on the left both while feeding and pumping (thank you Hadjare for this suggestion). I was absolutely amazed at the difference that this made while pumping. I often got an extra ounce or ounce and a half after I thought that the left had completely given up.
After a few days of this strategy (which was kind of a PITA, and probably wouldn't have been possible if I was working full-time), the left started producing WAY more. It was kind of amazing, actually.
So, I'm going to stop obsessing about my boobs and move on to something else now. Thanks for listening.