What Could Cause A Red Rash And Swelling Around My Ankle And Foot With No Visible Bites Or Bruises?

I was working outside in the heat. When I got home I took a shower and noticed that I had a red flat rash similar to razor burn around my ankle going up but stopping below my knee. The next morning the rash had moved down to around my foot only, but there is a large golf ball swollen area on the outer ankle. It is red but not really bruised. There is no visible bite marks, I don t recall hitting it or any reason it could be sprained.