The source that I consulted on this issue (The Nursing Mother's Companion, K.
Huggins) states that antihistamines can "Theoretically decrease milk supply"
 whereas decongestants, such as pseudophedrine "can decrease milk supply."
 It goes on to say that if you are going to use pseudophedrine, you should
choose one with 30 milligrams or less, and to avoid sustained release
products.  It warns to be aware of "a possible decrease in milk supply."

As for Antihistamines, she states that they can cause drowsiness in infants,
and that often a single does at bedtime is sufficient.

Anne