I'd slipped on some moss or slime, or whatever, that forms there in the autumn and winter months.
I got up, tried not to cry, and carried on. Later, I jetwashed the patio to try and remove whatever it was.
It swelled up a bit, and was really quite painful.
The following day was Pilates. I was OK for most of the class, but refused to do the floor exercises where I was lying on that side.
Today I coukd feel that the swelling had gone down, but it was still painful. DH suggested I looked in a mirror.