I could find no current information on Allegra (fexofenadine) and
breastfeeding.
It would probably be OK to use during breastfeeding considering the
general profile of the current meds on the market, BUT why not use the
nonsedating antihistamine, Claritin (loratadine) which has mimimal
excretion into breast milk and is unlikely to have an effect on the
infant.
Also, use only the Claritin (or Allegra) rather than the Claritin D or
Allegra D.  Add pseudopehedrine to the regimen only if the decongestant
is also needed in addition to the antihistamine.

Frank J. Nice, DPA, CPHP