My son is 5 months old and he has bright yellow poop and he over fills his diaper and in his diaper it looks like a big yellow puddle?

Loose stools: If your baby is breastfed, this is not unusual. But if not, he could just have eaten too much - more formula than normal or jarred fruits and oatmeal if already taking solids. Otherwise, it could be an infection if he gets more loose stools.

BM: Are you breast feeding your baby? You have described a very normal bowel movement for a breast fed baby.

Bright yellow poop: Bright yellow poop is not unusual especially if baby is being breastfed. The yellow puddle in the diaper likely stool mixed with urine. Blowout diaper (when diaper overfilled with stool) is not unusual. Follow up with pediatrician if loose stools continue- more than 6x in 24 hours or if vomiting more than 3 times in 24 hours.