It's so scary when our beloved animals are in trouble and we flounder about trying to find the cause. Just fyi. About 5 years ago my BMD had a debilitating illness. We thought at first it was Lyme disease, but my brilliant vet (who spent countless hours trying to diagnose him) finally came up with immune mediated poly arthritis and we followed a regime of antibiotics, doxycycline and prednisone for 8 or 10 weeks. Also, NO exercise for something like 4 months. I'm pleased to tell you that Baron recovered, although he never received another vacinnation in his entire life (he lived almost 10 years). This was a very frightful time in our lives, as we were afraid we'd have to put him down. He went lame in the course of 12-15 hours....
I wish you all the best and hope you can get to the bottom of this. If you need any more details, I'll be happy to research exactly what we ended up giving him. Post back and I'll go searching if need be.
Only good thoughts!
Sara