A friend of mine went to casualty yesterday with a swollen foot. She thought she must have injured it running in the park the previous day. (It was achey but not swollen after the run). But it turns out that she had in infection due to a "rub" she had got in her foot earlier in the week. The swelling needed to be drained and she got an anti-biotic injection, oral antibiotics to take for a week, a compression dressing and strict instructions to stay off her foot for while.
It struck me that it would be very easy to do this sort of thing, and not seek medical help because of wrongly assuming it was an injury that would go away with rest, ice and elevation. I suspect it felt hot to touch. I also suspect that left untreated it would have got very nasty indeed very quickly.
Just something to bear in mind - particularly if you're not aware of having injured yourself.