So, I think my sheep's cysts were caused by the nodule worm (Oesphagostomum columbianum).
I'll post back here if fecals confirm its presence, or suggest a more likely culprit, in my flock. Thank you, everyone!
Belated Update: full-flock fecal tests showed some worm load, with three scores of 0/g, the two youngest lambs scoring 50/g, their mother showing 225/g, and another ewe showing 125/g. Diarrhea was possibly a temporary result of switching to richer pasture. Will work on controlling worms and improving general body condition among the ewes.