Hi Lill, I bet if you can give it a good look over you will find some kind of bite mark, if so you just need some antihistomin cream to rub on it, but in the mean time I would find a pack of frozen peas rap the in a thin tea towel and reat the back of your knee on it, this will help cool it and get down the swelling, sounds just like and insect bite that you are allergic to.
If it still persists see your doctor.
Good Luck
JGoodman you are really good at giving a practical answers to solve the problem of members.
Your best bet is to call your doctor - most offices usually have an answering service or a telephone number for an "on call" physician. Explain your symptoms - they will know which questions to ask - and have them help you decide if it's an emergency situation or something that can wait for an office visit.
Swollen, red, hot & itchy - that could be anything from simple dermatitis or irritation from jeans ..... to a bad spider bite that makes the back of your knee look like a water balloon. I wouldn't want to draw any conclusions - but don't itch! That can cause an infection. Try a little benadryl spray in addition to a touch of hydrocortisone if it's just a rash. Otherwise, ice it and try to speak with a doctor.