Help! Strange black oval under my knee!?

I just found a weird black oval shaped thing under my knee. It is about 1mm wide and 1 1/2 tall. It was hard to remove. When it was squeezed, it hurt me but it did eventually come of with a little pulling. It left a small red mark, but nothing else. I live near west Chicago, but I was recently in Michigan, and I swam in the lake, just north west of Michigan city. What could the strange thing be, and what could it do? Oh, it had no appendages, and no oddities. It was solid shiny black