Hey, I spun off a cog

This afternoon, as I was zipping along on the way to a meeting, I heard a sound from the rear hub and suddenly the cranks weren't turning. I wasn't consciously slowing down so I don't know what caused it to spin off at that moment. Luckily enough, the chain didn't jam and I coasted to a stop. This is the first time I've had a cog come off in the several years (4? 5? 6? I can't remember) I've been riding fixed. I have both front and back brakes and never skid, although I do a fair bit of speed control, more so in winter. I put the cog back on, a grimy business, given the shocking filthiness of my bike, and carried on. I've now tightened the cog up with a chain whip but I suppose in future I should keep a closer eye on how tight the cog and lock ring are. In other news, I replaced my very worn chainring several weeks ago. I didn't have a 42 tooth ring so I put on a 43 which has raised the gearing to about 69 inches (43 x 17). Initially my commute was noticeably more tiring with the new gearing but I seem to have adjusted. It still feels harder but doesn't really tire me out. It's a bit high for days when there's a headwind or there's snow on the ground but I think it should be ideal spring arrives and I take off the studded tire on the front.