The only thing I can say is, when they are at this "just barely walking" stage, it can only take the slightest thing to really throw them off their game. It's like the slightest thing is greatly amplified. For example, there were a couple of times where I noticed that the first sign of a UTI in my dog, before any noticeable change in her urine, would be that her walking would go all to heck. Same thing with constipation. When she was learning to walk again, if she was constipated it made a difference in her walking. So I don't know with your dog, it could be something neurological or muscle strain, but it could easily be something very common as well. It's just that it takes so little to throw them off at this stage. It helps to keep the nails trimmed, especially the front. My dog used to split her nails because she was putting more weight on her front feet when she was learning to walk again. The therapist used to say that sometimes she had her "off days" in PT and we usually did not know why.
Congratulations on the walking!
