Swollen foot

A guinea pig belonging to my mother-in-law got a badly swollen foot today. The piggie in question is Almond, adopted a few months ago with Cashew and Hickory. Yesterday he had a very slightly swollen toe, but it did not appear sensitive to the touch. Today (yesterday I guess by now), however, the whole foot was swollen, with a large lump about the size of my knuckle. It appeared raw. The vet drained it, although it immediately swelled again.
Of course we got him over to our vet right away (fortunately our vet is open 24/7; twice we had to take our bun in and once our hamster during abnormal hours), and he is now on a regimen of hot compresses and antibiotics. However, I don't understand what could have caused this. They are kept on aspen shavings, no rough surfaces to walk on (except one small 'bridge' that is wire). Has anyone else had this sort of problem with their pigs before?