I purchased the book Medications and Mothers Milk (Paperback)
by Thomas W., Ph.D. Hale (Author) on the recommendation of my lactation consultant while breastfeeding my first child. I discovered that most doctors refer to the same manual when advising or prescribing medications to a breastfeeding mother. I have some nasty allergies and yes, in my experience, antihistamines can partially decrease your milk production temporarily. I am considered a "good milk producer" (once we figure out the latch problems), so my milk production only suffered for about a day even when I stayed on the medication for weeks. Have you tried plain Claritin or something along those lines? They didn't seem to cause as much issue with my milk as those with the D (ex-Claritin D).
I hope that helps, but I definitely recommend that book. I even carried it with me to doctor's appointments to be sure we checked on whatever medications I may have to take.