My eldest suffers really bad with seasonal allergies (hayfever) and has done from a young age. During the worst months he's on a daily dose of antihistamine
I'm not sure what the active ingredient is in Zyrtec but you may find that it might not agree with her, the first antihistamines we had made DS very grumpy and drowsy, we then switched to a different type and he's fine with them. If she seems out of sorts don't hesitate to ask for a different type.
I hope they help, allergies suck!