4 mo boy exclusive breastfeeding. has rashes on face every time when mom eats prawns or shell seafood. allergy? coincidence?

LIKELY: If the kid breaks out each time after exposure to BM influenced by these items, an allergy to some of the proteins in the seafood is possible. This can be confirmed by allergy testing, but I would avoid the foods for now.

No: Since breast milk does deliver some food allergens consumed by the mother, it is not a coincidence if this happens every time you ate shellfish. I would suggest that you avoid shellfish or discontinue breastfeeding. At age 4 months, it is also time to introduce table foods (no shellfish) to the child's diet since this may actually reduce the risk of developing yet more food allergies.