
Hi Jan,
Sorry about your girls, hope Penny gets better soon.
Yes, I think it could possibly be the spine; when Spender slipped his disc, he would get an attack every so often where he cry out, haunch himself in his bed, jaws clenched, eyes wide, then they would dull, he would lie for a few minutes and then he'd get up and he seemed ok albeit a bit subdued. With him, he wasn't using his tail like he normally does and as he walked, he was holding his spine rigid, it wasn't flexing with movement.
When the disc finally protruded, it compressed the spinal cord which affected his rear movement.
If the abnormalities in the spine are compressing nerves, nerve pain can be excruciating and that in turn can cause a spasm in the surrounding muscles.