Suggest Treatment For Swollen Welt After Bitten Mark

Hi, I've detailed this incident because I can't tell if I've been bitten or stung by something. Yesterday afternoon I was stung or bit on the top of my right foot. I live on the western slopes of the Northern Cascades in WA and yesterday was the first sunny/warm day this year. I was cleaning off the porch from winter wood, etc. then stepped out of my slip on shoes. When I slipped them back on I immediately felt a pin stick followed by a burning sensation and a nickel size welt. At first I thought a vein maybe popped. I continued with yard work but couldn't put a shoe on that foot because of the pain and swelling. A while later a headache began (which I attributed to being in the sun so long) and still have, I was limping and there was bruising around the red welt. Last night the swelling and bruising spread to my ankle. This morning I had bruise-like discoloration extendeding up to the very lower front of my leg, about 6 inches from bite/sting area. The welt has gone down some but is wider in diameter, kind of hard and very tender to the touch. It is red with some yellowing around it and bruising covers most of the top of my foot. My leg has an achy feel half-way up my leg, still can't put a shoe on and still limp. All we see is a very tiny single puncture mark in the welt. I thought perhaps a Hobo spider got in my shoe but I didn't develop the trademark bulls-eye. Could a centipede do this? Thank you for any help.