We get this in the commercials from time to time. It always rights itself once the sun comes out.
This year I had it in two of my fleece flock lambs. One was a triplet, to a very experienced mother. Rosie Mule kept it moving, didn't let it curl up and stay put, and it came right within a few days. The other mother was a first-timer, let the lamb stay put, and the lamb wasn't improving. The mother was short of milk for two anyway, and I had a lone pet lamb, so I brought that one in. 36 hours in the warm, lots of exercise as its pen-mate is a bully!, and it was much improved. I'd been prepared to do tendon-stretching physio with it, but it wasn't required.