I am ashamed to admit that in my first year, I had a ewe that did the same thing as this, after lambing (that afternoon in fact) her poo loosened and she gradually went down. Treated by vet for pretty much everything under the sun, but suspected worm overload. She was dead by the end of the next day.
I had planned to worm once she had lambed. I now worm two week pre-lambing, or at the point that they go into the "lambing field".
If they weren't wormed pre-lambing then worm them now.