I started taking zyrtec (cetirizine) for my allergies & it makes me very thirsty, is that normal?

Yes. Antihistamines are used to "dry" up secretions. It is one of the reasons that they help (minimally) for allergic noses and try to help stop the runny nose or sneezing or post nasal drip. Its main mode of action is to dry....So it goes along that taking an antihistamine can cause dry mouth or increase your thirst. Try taking half the dose.