I Have A Pea Sized Lump On The Joint Of

I have a pea sized lump on the joint of my middle finger on my right hand. It is a little tender if I push on it but not really painful. It can move slightly if I try to wiggle it. I think originally I got it when I was painting a room about 2 months ago but it has got larger after I was cleaning up a barn by sweeping it up. Any ideas what it is?