What Causes Sting And Bruising On Arm?

I was boating on a lake yesterday in Northern CA and all of a sudden I touched my right forearm and felt like I might have gotten stung or bitten by something. I did not feel it actually happen, just apparently shortly after the fact. I thought maybe a stinger was still in my arm because when I touched it, it felt like a needle or something sharp. It has changed looks since then. It started out small and white in the middle with just a little reddish circle around it. Then within an hour it was turning purplish/red and was the size of a dime. When I woke up this morning it is now the size of a quarter and looks like a bruise. There is a white dot in the middle of the bruise, where I assume the stinger or bite was. Does anyone know what this is? It is not raised like a mosquito bite and does not itch. Thanks!