eta I was starting to wonder if maybe this was some weird side effect of my meds, although I haven't changed them apart from experimenting (with the doctor's advice) with the dosages lately. But then I came into work, broke down a little (people are still giving me a wide berth...) and monkey_matt suggested it might be an ear infection. Trust me to go for the complex and unlikely explanation when the simplest one is probably right.
I was on the stairs in the underground, went straight down adn the gasp from the woman behind me on the stairs made me think my leg was turned backwards or something. (It wasn't, I landed on my elbow of all things...)