I have to agree - that rough circular patch does sound like a corn. And they can certainly be painful and cause the increased limping that you describe. I was convinced that one of my fosters had "something" going on - her limp didn't improve with anti inflammatories, x-rays were clear, even went for chiro treatments. Nothing worked - until we discovered the corn on the pad of her foot - the vet had missed this as well.
Check this forum for some of the threads on corns and removal options. I'm sure glad I did. My girl is now living comfortably - most the time that is.