This happened to me about two years ago. I was hanging out on the back porch barefoot. A couple hours go by and my foot started to swell. I tried to put on sandals to run to the store and they wouldn't fit. My foot just kept getting bigger and bigger. The man had to carry me to the ED that night because I wouldn't walk on it. I got prophylactic antibiotics and a small dose of steroids for the swelling (both IV). The next morning I woke up and my foot was back to it's normal size.
Go to the ED because urgent care may have the meds stocked for adults but not necessarily for peds.