Why is my black lab unsteady on all 4 legs? (her arthritis is minimal)

My dog is a black lab, 10 years old, and weighs 77 lbs. She has been showing unsteadiness and weakness in all four legs. She is unable to support her own weight and she slides on smooth floors. She's obviously in pain. She would cry upon standing up from laying down and now needs our assistance to stand up. She didn't really want to move at all (even to ask for food, which is odd for the always-hungry lab!) and was pretty unresponsive to us.

We took her to the vet and he ran extensive blood, lyme, and tic tests. All tests came out negative and within normal range. He identified her arthritis as mild and only in one back leg. He prescribed her Previcox, which helped the first few days. She became visibly happier and more active.

After 4 pills (one per day), she has completely lost her balance.

Her vet was puzzled by this condition - any ideas what could be afflicting her?

Also, throughout this time, she has continued to eat all of her food, still drinks water, and is making

Asked by Member 1126362 on Aug 18th 2012 Tagged hindlegs, frontlegs, unsteady, tired, unresponsive, sad in Health & Wellness