vet is the answer of course, but I wonder - could it be Joint-ill... I only mention it in case you've seen joint-ill lambs before and it jogs your memory. I think back 10 years ago is was common to use penstrep (from the vet) injection for this illness. Can't remember what the success rate was though.
we once had a lamb with some sort of nerve complaint that exhibited similar symptoms, she turned into a perfectly healthy sheep, but she had a pronouned "hunch" in the lower spine/pelvis region.