Coughing in the sense like trying to expectorate or just the sound of cough? In case it is just the sound, it may be due to nose block which is very common in infants, in case of expectoration you need to see a Pediatrician especially if it is associated with fever, feeding difficulties and poor weight gain. Any upper respiratory obstruction also needs to be ruled out. So pay a visit to your Pediatrician and let him see what exactly is the case.