"You'd have a very odd circulatory system if you'd only got one swollen foot as the result of a heart problem!"
- That's a quote from my doctor when I went to see him with one swollen foot (just like yours) a few years ago. He diagnosed an infection (which can easily get in, even if you're unaware of any breaks in the skin). He prescribed a course of antibiotics, which resulted in near-immediate improvement, but there was still some residual swelling and 'hotness' when I'd finished the tablets, so I went back to the surgery. He said that it was quite common for foot infections to be slow to clear, so he prescribed more antibiotics. At the end of that course, I still wasn't entirely convinced that the problem had been fully dealt with, so I returned to get a third course of the treatment (which, finally, did the trick).
So you do need to consult a doctor (perhaps at your local walk-in centre?) but there's no great need for panic.
I hope, however, that the infection isn't also affecting your memory. The bank holiday is NEXT weekend! ;-)
Chris