In general, no. Pepto Bismol contains bismuth subsalicylate. Salicylates are not recommended for children under the age of 12 because there is a link to Reye's Syndrome associated with using them. This is the same category of medicine as aspirin, and the Reye's Syndrome risk is the reason children generally do not use aspirin products.
While I cannot see Pepto causing a problem, you are typically not supposed to take anything while you are breastfeeding. So, to be safe, I would ask your doctor.