[SonicPs]

Have you been in or around any bushes lately? It could be poison oak or poison sumac. It's a bit late in the season, usually the frost kills the leaves, but the oil from those leaves can last a long time. In 1998 I hiked the John Muir Trail in the high Sierras during the early spring and one day I sat on a fallen log in shorts to eat my lunch. A couple hours later the backs of my calves had one of the worst poison oak outbreaks of my life. I can only guess that last summer it was covered in poison oak, that the plant died during the winter snows, but that the oil had soaked into the wood so it was still there when I sat on it.