If they are strung together they are often strung by fur-it’s usually during high shed (Which-it is that time of year). I doubt it’s the grass, but I’d re-double your brushing efforts and add more hay-that should help. It is normal-we all see those around shedding times

She may be producing extra cecals too as at some point the babies will start to eat them. They won’t be making their own yet. I’ve read this somewhere but have forgotten. If I find it again, I’ll post link here.  Are you giving her unlimited pellets?  With mothers – they need extra food to produce the milk. 

Edit:  http://www.rabbit.org/care/babies.html#Fee…by_Rabbits This should open to relevant part of article.  4th paragraph down.  You may be seeing some cecal mixed with regular poop – especially if it was greenish, shiny and smelly(?).  I do know with my rabbits when there greens have increased from veg or grass the poop is darker.