Fexofenadine is one of the preferred antihistamines or allergy medications that mothers can use while breastfeeding. Low levels are found in the milk, meaning no sedation or other effects would be expected in the child.
Like any antihistamine, there is a chance that the milk production could be reduced. Consult your doctor or a lactation specialist if that is a concern.
To help answer all your questions about medications and breastfeeding, I’d encourage you to get a copy of Medications and Mother’s Milk: A Manual of Lactational Pharmacology. This is a great reference to answer many questions about herbs, medications, vaccines and breast milk.
Always as your doctor before taking any medication while breastfeeding. Contact your pharmacist or doctor if you believe you or your child may be having side effects from medications.