Could be - it's certainly worth looking at as they are the prime age for it.
It is caused by a parasite and the scour is usually dark and sometimes bloody. Have they been housed since birth? It is usually picked up at grass. Most sheep can tolerate low levels of coccidia infection but young lambs are obviously more susceptible. Relatively easy to treat with an approporiate dose, but it will set the lambs back a bit.
Of course, it may not be that at all.....