As far as Zyrtec, it has a drying agent, and that can dry many fluids in the body... not just the nasal passages and sinuses, and could have certainly played a part in your breastfeeding supply. If you speak with a lactation consultant, most will tell you that until your supply is established, it can be tough to produce with pumping only. The best demand is created by the baby, and not a pump. In the first few weeks postpartum, it's best to try as often as you can to put the baby to your breast and have them latch to establish a supply. If you want to have milk storage, one solution to bild a supply is to pump on the other side while the baby is nursing (their stimulation causes a let down (or multiple let downs) and you would then get more milk than pumping alone). If, for whatever reason, you cannot nurse the baby, and only pump, that too could have an effect on your supply. It also depends on if you are feeding on a schedule (or pumping on a schedule) instead of feeding on demand (which has been known to diminish and eventually dry up a milk supply).
In response to someone that said Dr.'s would not give you something that would harm you or your nursing baby, that's not always the case, because many Dr's are not completely versed in what is safe or not safe while breastfeeding. It is ultimately your responsability to find out if what you are taking would get into your milk supply and get to your baby, even in minimal amounts.
I wish you much breastfeeding success with baby #2!