Gastro-colic reflex
When a baby sharply pulls away from the breast or bottle, fidgets, grunts, passes some gas or a poop and then returns to feeding, this is due to the gastro-colic reflex. This is normal infant behavior! Feeding stimulates the gastro-colic reflex, which triggers an urge to poop. Sometimes a baby will poop, sometimes pass gas, other times there will be nothing to show for his efforts. When observed during feeding this reflex is often mistaken as a sign of pain.
Hypersensitivity of the gastro-colic reflex, which presents as frequent spasms which occur in waves that continue on and off for hours, is one of the many theories on causes of infant colic in babies younger than 4 months.