Get it checked out. I had a neighbor get bit by something while he was doing some yardwork, which produced a welt the size of a softball in the kidney region of his gut. It wasn't pretty to look at let me tell you.
... When I was about 22, I hit a beekeeper's beehive on a warm summer day with a gas mower while wearing a bright colored shirt. I'll let y'all picture in your minds what happened next. Surprisingly, I still don't fear bees. Never have. Some spiders, yes; bees, no.
I also thought that a black widow's bite really doesn't live up to the hype. It might -- MIGHT -- get you sick, but death, no.