My 7 year old son started taking Zyrtec 5 days ago. For the past...

My 7 year old son started taking Zyrtec 5 days ago. For the past couple of days he has started acting crazy. He has outburst where he literally runs through the house waving his arms over his head screaming at the top of his voice. He has never in his life acted this way. He also will not obey. He will be doing things to annoy someone and when asked to stop he SMILES at you. It is really very frightening to see him act this way. He is usually such a sweet and compassionate chld.