I Developed A Red Itchy Rash On Both Wrists And

I developed a red itchy rash on both wrists and palms and top of hands. Very red, flat, serve itching,hot and hands and wrists are swollen. Also found it behind knees but not as itchy there and doesn't hurt. Fingers very swollen too and skin is tight. Aching in wrists and joints of fingers. Benadryl and hydrocortisone cream are not helping at all. What should I do?