What could it be if my feet have suddenly swollen with small red-brown rash like dots that seem to be in a cluster on the top of my foot and calf?

Many possibilities: There are several different things this could be. You really should see a dermatologist and have a proper evaluation.

Rash: This can't be diagnosed with the information that is available. A history, physical examination & other studies may be needed to determine cause/causes. Once accurately assessed a treatment plan can be developed.