She needs to go to an ER, not an urgent care. That is bad.
FWIW, my son got a mosquito bite that blew up about 3 inches in diameter but quickly settled back down. He has a darkened spot still, 2 weeks later, that is about the size of a quarter. He seems to be allergic to mosquitoes. If his leg had looked like THAT, though, I would have run him to the ER immediately.