What Could Cause Vomiting And Diarrhea After Treating Cold With Azithrol?

Hello Doctor, My 2 year old son caught cold n cough and his doctor prescribed antibiotic Azithrol 100 for 3 days. After the completion of the course he started passing loose stools and vomitting. At first we thought it might be the congestion, the reason for cough, which he is passing out this way as he cannot spit it out like adults. But today he vomitted total food each time I fed him for abt 4 times. And his stool is very watery and passed it for more than 5 times today. Pls suggest the immediate action. Thanks in advance