my son is 15 weeks old and about a week ago he ...

my son is 15 weeks old and about a week ago he started drinking funny. he normally used to take 200ml of formula no problem and this was the recommended amount for a baby his age, then he just started refusing to finish with about 50ml to go. he is still going 3.5-4 hours between feeds which was the same as before, but just wanted to know if this happened to anyone else before?? is this a normal thing for a baby his age?? i'm just getting worried that he won't be getting enough food or possible start losing weight. he still looks well and seems very happy. if anyone has any info on this that would be very helpful!!