This is a very common sypmton of oxalate dumping. I work with a lot of mommas who have to eliminate oxalates from their and their children's diets. 
From this blog:
"Then this past spring (after nearly 18 months of GAPS) little J began to tell me that it hurt when she peed. She had already turned 3 and I had been wondering when she would ever master potty learning. When I learned that it hurt to pee (a classic sign of oxalate issues) and I realized that her poopy diapers often looked like they were full of beach sand (another classic sign) I decided to really understand oxalates and commit to a low oxalate diet (LOD) for her sake and my own."
http://www.lovingourguts.com/what-are-oxalates-2/
Jennifer Tow, IBCLC



Date:    Thu, 27 Mar 2014 16:21:38 +0000
From:    Sarah McCann <[log in to unmask]>
Subject: Grit in poo

The mum described it as feeling like coarse sand and it wasn't crushable. The 
nappy idea is interesting.Thanks again,Sarah McCann. 		 	   		  
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Date:    Thu, 27 Mar 2014 14:16:11 -0400
From:    "Kerry N Zenich, RN, BC Peds, IBCLC" <[log in to unmask]>
Subject: Grit in Poo

Hello Sarah.  I have seen this several (3-4) in my nursing career, and as 
someone just stated, it was the diaper.  The first time many years ago, it 
startled me enough to show a few other nurses, one of whom was very wise and 
stated right away that it was a "poor" diaper.  You  might suggest this to the  
mother since intake and output sound WNL.  Maybe try a new type of 
diaper...maybe she changed recently?  I doubt it was leaving a soiled diaper on 
too long, probably just not the best of diapers.  Good luck and keep us posted!



 



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