Seasonal allergies and food intolerances can increase in the post-partum period. There is no specific explanation for why this happens. More research is needed in this area.
Many changes happen in the body during pregnancy. This includes hormonal changes that return to normal after the birth of the infant. There does not appear to be an increase in the actual development of new food allergies. Reactions due to food intolerances (e.g., non-immune, adverse reactions to foods) can increase.
Discuss your symptoms with your healthcare provider. If there is a concern you have developed a food allergy, meet with an allergist. This could be very useful in diagnosing a food allergy versus a food intolerance.