Bizarre insect sting, what on earth stung me? Scorpion? Itching/swelling/numbness?

Ok I got stung by something the other day down at the pool. I am not sure what it was all I got was a look at the stinger because it was stuck in the bottom of my foot. I live in the desert area of Southern California (San Bernadino County). Here it is 2 days later and my foot is still slowly swelling, I am pretty confident I got the remaining pieces of the stinger out. However my foot is still swelling, itching, and a bit numb. I don't have any other signs or symptoms so I am hesitant to make an expensive trip to the dr's office. I am curious though on what it is thats out here that can sting like that. I am trying to figure out if it shot its stinger out at me, or just casually came up to my foot and stung it. I was sitting in a chair, and saw nothing crawling around after it happened. The stinger appeared to be a grayish color? I was in a lot of pain so not really paying that close of attention to details lol. Any help would be greatly appreciated! Thanks!