Our 9 yr old border collie mutt started limping w/o apparant cause about 10 days ago. We find no visible injury to pads, between toes, nails, foot or leg. But he is not happy about us touching and examining that front foot. We've cut out walks and running in the yard, so he seemed to be getting better. But it flares up off and on, especially outside on uneven surfaces like rocks and grassy areas. He doesn't like it or chew at it, and there is no swelling or oozing. He just limps off and on. Any suggestions?