Thank you for asking the query.
Loose stools or diarrhoea is a known side effect of Cetrizine (Zyrtec). Although it is not common, the pattern of loose stools you are having points towards a possible side effect of the drug.
So if the loose stools are troublesome you should probably stop taking it. You can try other anti allergic medications, but they also might cause the same side effect (because of the anti allergic action). So the best approach would be to see if you can do without it by using other means like nasal spray.
Hope this helps.
Please feel free to ask followup questions.