It seems that I sprained my ankle yesterday; not when I was walking though, but when I was struggling to get up out of my seat on the train.
As usual, after a day's walking I was tired and ached all over, and after already travelling for an hour on the bus I was very stiff and had limited mobility. I had managed to find a seat, one where there was a table, and sat down. Because of the lack of space my feet were sticking out into the aisle by a few inches and so when a lot of the people got on at the Meadowhall shopping centre with their bags I tried to move my feet out of the way. Unfortunately they ended up being entangled with the table leg and the strap/handle of one of the bags that was on the floor. When the train reached Doncaster my right foot was trapped and I had to twist it in an awkward way to release it.
At the time, possibly because of my other aches and pains, I didn't notice anything, but a few hours later when I got up from sitting in front of the computer I was aware that something was wrong.
Fortunately the sprain doesn't seem to be too bad, there's no bruising or swelling - so I'm hoping a few days rest and wearing a support bandage will suffice.