Baby stops feeding after switching breasts

My baby is now 11 days old. A couple days ago he used to feed about 30 minutes on both breasts but yesterday and today he stops feeding after switching breasts, which is only 15 min. We’ve tried many things, e.g., burping, tickling his lips, ears, feet, etc but he is just not interested and sleeps. He doesn’t cry though. I’m a bit worried that he can’t get enough milk since we feed him every 3.5h now at night (2.5h during the day).

Should this be a concern? How do I know how much milk he has each time and if he has enough? Should we increase the feeding frequency at night? Could this be caused by using pacifier?

Thanks so much in advance!