If it was something that bit you it might be that pica pica stuff, it not, it could be something I catch frequently in Jamaica called Sea Scratch. When in Jamaica after a rain you tended to see a lot of sea grass in the water and for some reason or the other it left this residue that caused a reaction in my body. Don't laugh, but I was told to poor rum all over my body. I figured what the hell, if it didn't work I could drink it, well it did (I also think straight rubbing alcohol could work). The end result was that it did go away but scratching it made it worse, an antibacterial lotion worked, Neosporin I think. Good luck and try not to scratch.