Baby losing weight

Stephanie • 💙💗💔
This is week two back to work. I pump 3 times during a 7 hour period. My son is 12 weeks old. 4 days ago my son was weighed 13 lbs 7 ounces. Tonight he weighed 13 lbs 4 ounces. Is this something that should worry me? I've notice that he goes longer without eating lately. :| Could this be due to not being able to nurse him but rather pumping during the day?