Sudden appearance of red itchy rash behind my knee, swelling and spreading. I scratched my eyelid and got four bumps there too so its spreading?

Allergies. Patients with allergies can develop red and itchy skin around the eyes and on the eyelids from allergies. Behind the knees and on the inside part of the elbow too.This skin condition is called allergic dermatitis or allergic eczema. You can use a hypoallergenic cream/lotion: cetaphil or lubriderm and consider contacting your medical doctor about taking allergy pills-such as Claritin (loratadine) or allegra.
Have you been. Outdoors? Sounds like a contact dermatitis, such as poison ivy! see a doctor if the Benadryl (diphenhydramine) isn't working.