Mystery Bruise

A week or two ago I noticed I had a scratch on my arm. It didn't seem to damage the skin, it just left a red mark about an inch long. Underneath the mark my arm was swollen. There was a lump about a quarter inch tall and two inches in diameter. I just looked at it again today and the lump is almost undetectable, but there is now a bruise in the same place. All I can think is that it must be some kind of allergic reaction to some creepy plant, a plant that everyone else probably loves. But to me, it's creepy and I don't like it, whatever it is.

Update February 2017 replaced missing picture.