[ Horses ] Open Question : swelling on horse's elbow?

My horse has a tennis ball sized area of swelling on his elbow. It is hot to touch and he flinches slightly if I touch the area, but he is not lame. It is slightly firm, but not rock hard. I've been searching online to try and figure out what this is and it sounds like a shoe boil. However, my horse is currently barefoot and is mainly in the pasture. He is stalled only at feeding time. I rarely, if ever, see him lying down in the pasture. If he's down, it's because he is rolling. He certainly doesn't spend long periods of time lying on the ground. Today is the first day I've noticed the swelling, and I hosed it with cold water and applied a topical antibacterial ointment. Could this be something other than a shoe boil? Should I call the vet now or keep doing what I'm doing and only call if it doesn't get better after a week or so? Thanks!