What Causes Clay Colored Stools In A Child?

I have an 11 month old who has been constipated for 3 days, he does poop but it is very strained and the feces are clay like balls. Now this is from a baby who up until 3 days pooped on avg 3 times a day no problem. I have noticed these tiny clear yellow balls filled with fluid because I can burst them. What could this be?