What could be the side effects of a small cetirizine and diazepam overdose?

Increase of sedation. Cetirizine is a sedating antihistamine and when added to diazepam/valium (a hypnotic) can compound the effect of sedation. If just 2-3x the usual dose may not cause much more than prolonged sedation, but if excessive od of diazepam and cause respiratory depression/low oxygen which may trigger hypoxic injury to your brain/heart. Be carefull, be safe, take meds only as given. Consult doc. Good luck.