Why might an exclusively breastfed three month old infant who poops 4 times a day suddenly not poop for 24 hrs even without any change in diet?

Breast babies can do a little smear every feed or a cradle filler every 7-10 days. Breast milk is a well-designed food source compared to formula, where more debris is left over after processing. I would not assume the change in pattern has any meaning. If the kid doesn't go in 3-5 days, a little Vaseline coated q-tip inserted in the anus could stimulate an evacuation. If it is still soft, ignore

Dr Kuitems agreed

As an infant’s, gastrointestinal tract matures through the first 2 to 6 months of life there can be a lot less waste. It is not average, but not rare for an infant this age to have only one or two bowel movements a week. They should never be firm or uncomfortably hard bowel movements, but they can be very infrequent and still be very very normal.