Mine are the same, and its not worms (never had any, and had plenty of worm counts done), they have been treated for fluke in late winter, and they have MG licks too. However my field is full of clover, and the goats are also quite loose, but not runny. I think its the lush grass/clover. Have dagged all my ewes prior to coming out with lambs, still all gunged up at their backends....
Just a word of warning, I didn't initially feed when the ewes went out wiht their lambs (licks were there though) and I had a case a Mg or Ca deficiency/grass staggers, which I managed to improve with injection and feed! but have now stopped to feed concentrate.