What Causes Red And Swollen Knee With Dizziness And Aching?

Husbands knee is red and swollen. He did not bump/bruise it. I believe that it could be a horsefly bite. He has some dizzyness and achy-ness. I gave him some allery meds and had him put a cream on the knee. I also gave him an ace bandage to wrap it with. We dont have much money and since it is Sunday the only available dr. would be the ER. Any advise?