I know this sounds weird, and I'm not sure how old your dog is, but I have a German Shepherd (at work), and he started limping really badly when he was about 12-18 months old. I thought for sure it was his foot, as he wouldn't walk on it at all, but couldn't find ANYTHING stuck in it, or other sign of trauma. My boss had me take him to the Vet, and they told me that it was a syndrome that young large breed dogs get, that affects their joints , like growing pains, And that it wasn't his foot, but actually his shoulder. They said it will move from leg to leg, front to back, appearing suddenly, at odd times. They gave him some painkillers, and told us to keep him quiet and confined, and within a week, it had gone away like magic. Every now and then, he'll have a strange ache, but like they said, it shows up suddenly, then goes away. We were given actual dog prescription meds for that, I forget what they were called, but I recognized the name from when my animals were neutered/spayed. It was the same kind of prescription for pain.
On another note, I give my 12 yr old 100# Lab/Pit mix buffered aspirin called Ascriptin, one pill when his arthritis is really hurting him. Its doesn't upset his stomach, and he usually has a sensitive stomach too (due to two gastrointestinal surgeries to remove inhaled items).
I hope your baby is okay. I've enjoyed seeing the pics you've posted of your dogs here before. They are beautiful.