Help!

After Lady finally recovered from her operation and learnt to move around really well on 3 legs, she now faces a new complication.

A couple of days ago, Lady started showing a sudden weakness in her remaining rear leg and I noticed that her gait had changed from a tri-pod look (with her remaining back leg being placed on the ground between her hips) to a 4 legged look (she now places it on the ground at the side as she would do if she had 4 legs). Yesterday, it gradually got worse to a point where she could barely hold her self up when moving. I rang my vet last night, checked for ticks and took her in this morning thinking she had either ruptured a cruciate or had the start of Wobbler's disease, both of which my vet had said were possible. However, once he had seen her this morning, he ruled out both conditions and also added that her muscle tone was good, spine was straight, she is not exhibiting any pain, she can support herself while standing still and that she had probably just strained a muscle and needs to be confined for a couple of weeks, given anti-inflammatories, heat packs and taken for a visit to a dog physio. I have an old cot sheet that I am using as a sling to support her rear end, but it is really upsetting me watching her look so helpless. My husband wants to send her to doggy heaven, but I want to see whether she improves. Now I don't know what to do?! I am so sad, distressed and upset. Has anyone else experienced this kind of problem?