red rash around bite

Hello all,
Last evening I was bitten by something. I'm not quite sure what it was but I was bitten on the top of my right foot and have no idea when it happened. There is a red rash around the area about the size of a quarter. In the center there are what appears to be 3 or 4 raised welts, one of which is brown. The area around the bites is very tender to the touch and even walking causes me slight pain, I am presuming from the skin stretching while I walk. For your viewing pleasure I have included some images on a page of my website. Thank you very much for any assistance you can provide, I don't know what to do!

http://paeremarkt.com/insectbite

Thanks again!