Dosage of Carbaxefed DM for infants

What are the side effects if any for babies taking this medication at the age of four months. I have a child who is four months of age and is taking the medication. Since that time the child has been experiencing abdominal pain, constant crying, lack of sleep and is not eating normal (does not like to drink his juice nor his normal ounces needed for the day). If any allergic reactions what are some symptoms? Instructions for the drug indicated that it may take several days to adjust to this medication and the time the child has taken this medication has been about that time. Reason medication was given the the BA was for a cough. Any information would be helpful.