maybe its from them. i have seen one or two of them trying to do that.I can't tell much from that pic either. Do you have a young cockerel after them? Once in a while, when they are just getting started, the young boys will grab low and often, causing sores that result in nasty looking bumps but it really doesn't last long and the swelling goes down in about 2 weeks.
As mentioned, I've never seen the pox....hope it's the same problem as mine, which is no problem at all.
Good luck!