well, almost all drugs taken safely during preg. are safe during nursing and I took Zyrtec during my 1st preg and while nursing my 1st baby as well as for my second preg. (preg. now)
I remember my dr. telling me when she prescribed it during my 1st preg. that it is a catagory "B" drug for preg. and nursing (which means it is very safe as virtually no drugs are given catagoy "A").
Hope this helps ease your worries!