4mths old baby fussy in drinking milk

My LO is now 4 months and he is fussy in drinking milk. we are changed from feeding 120ml to 150ml / 3hrs recently as we know most of baby now should be fed 150ml. However, my baby sometimes cannot even finish 120ml and it takes a long time for him to drink. We did not change the formula milk brand as well, and are unsure why is he behaving like this for about 1week +. He recently have stuffy nose and flu. could that be the reason why he finds it uncomfortable to drink? Im worried that he is not taking in enough milk as per his age, at 4mths now, compared to the guidelines and other baby.