Sounds like they suspect a tick disease since they are treating your dog with Doxycycline. A young dog with sudden onset lameness and swollen joints, the things I would suspect are:
*tick diseases (Lyme, Rocky Mountain Spotted Fever, Erlichosis)
*Neurologic Disease
*Autoimmune Illness (Rheumatoid, Lupus)

If the initial bloodwork was normal, I probably would submit a tick titer, and place the dog on steroids (Prednisone) and doxycycline and see how the dog did after a few days, which sounds like what you vet is doing. If the dog isn't doing worse, have patience and give the meds time to work.

Keep us updated.