Redness, Itching On Skin. Hot Area, Swelling. Have Arthritis In Knee

I had a large red patch of very itchy skin appear on my knee , a lot of itching and scratching and my knee was very hot, red and swollen. I do have arthritis in this knee as well as other places. I have also recently experienced some sharp, stabbing type of pains in this knee recently. At first I thought it was an insect bite, but can not see any sign of a bite at all. It has now gone red and blotchy and the knee remains a little swollen, it also shows a couple of bruises which I cannot acccount for. I would be very grateful for any help you can give me. I am a female and I was wearing trousers at the time, so am convinced there was no insect bite. Many thanks.