There are a couple of things that could possibly cause this. One is you possibly have an infection in your foot from something that has bitten you or something that you have stepped on. This is not an allergic reaction because you would have it in both feet. Also there is a possibility you could have an ingrowing wart on the bottom of the foot. The discomfort you are feeling is from the swelling itself. There is also the fact you may have twisted your foot and not notice it right away. When you push in the top of your foot does in make an indentation? If so your are retaining fluid. You need to see a Dr about this because it can be serious. Until you can see a Dr elevate that foot as much as possible and keep ice on it. Also stay off that foot.
An allergic reaction to something touch or bit see a doctor who might prescribe something to help keep the swelling down