Why did my dog die?

I had a 6 yr old lab mix. November 2010 he started becoming lethargic, limping changing from front to back legs, would yelp in pain if you accidentally touched him, had to be helped up to his feet when laying down, and had fever, legs sometimes shook when standing. Had bloodwork done, x-rays, joint tap, blood test for Lyme's Disease which came back positive for antibodies (we live in Florida and thought that was rare). Was treated for 3 weeks of antibiotics and he seemed to return to his normal self. A few weeks ago he started to show signs of no appetite, lethargy, so he was started on antibiotics again. He got worse with fever, bounding heart rate, pale inside ears, walked slow with head low and kind of swaying from side to side (head), clear liquid dripping from nose, panting...more bloodwork done and different antibiotics were given. Bloodwork came back with white blood cell count double, resting cortisol low, given test for Addison's Disease which came back negative. Also given new Lymes Disease test-negative as well. Started to get better again for a few days, then suddenly passed away. Any thoughts on what may have happened or do you think there was more I should have done? He had been to 2 different vets and 1 specialty hospital.