Hello,
Thanks for posting your query.

Gray stool can be caused by a few factors. In many cases, a baby’s stool changes colors because of dyes in your food or a change in diet of mother . Gray stool can occur if a baby begins eating more dairy products, such as when you wean him off of formula and onto dairy milk.But this is not the reason as your child is breastfed.Sometimes Such change in color can also be the result of taking antacids, which doctors sometimes prescribe for babies who suffer from severe heartburn, also known as gastroesophageal reflux disease.In rare cases gray stool is a sign that there’s a problem with the baby’s liver. For instance, this change in color may be caused by a serious childhood liver disease known as biliary atresia.

As of now if your baby has one instance of gray stool but seems otherwise fine, something in her previous meal may have simply caused the change.
However If your newborn appears jaundiced( yellow) and has dark colored urine in addition to gray stool, these may be signs of a liver problem.

Please do not worry as I assume that your breastfed baby has no other problems ( since you didn't mention) and most likely it's just a variation of his normal stool ( either induced by diet or medications or multivitamin etc).
So you need not be concerned unless there are any new symptoms like yellow sclera or dark yellow urine etc or persistent gray stools beyond one week , in which case you should get your baby evaluated by a pediatrician promptly .

Hope I have answered your query.
Kindly accept my answer if you have no further queries.
Wishing your baby good health.
Regards

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