I Have A 1 Year Old And She Was Having

I have a 1 year old and she was having problems with allergies so the pediatrician put her on Zyrtec and she has been on it for a little over 3 weeks now. A week and a half into taking it she was becoming very irriatable and not sleeping very well. I skipped a day last week to see if it may be the medicine and she was perfect, so I thought I would skip another day to see if it was the same. I skipped yesterday and today and she has been perfect. Is this a common side effect to this medication?