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With the initiation of dietary solids, the intestinal flora rapidly converts to "adult-type" bacterial species, hence the change in smell. Second, the gas may also be the consequence of bacterial breakdown in the large intestine of unabsorbed starches from the cereal. Complete development of starch absorptive function is not achieved until approximately six months of age, when pancreatic function reaches maturity.
Finally, breastfed infants commonly experience a significant reduction in stool frequency by three to four months of age (another normal event). This fact, along with the production of a more formed BM (because of the addition of solid foods in the diet), may explain your baby's reduced stool frequency.
In the summer months, reduced stool frequency may also indicate the need for increased fluid intake. You might try adding more breastmilk to the cereal. Of course, if you are still concerned, check with your pediatrician.