Definitely normal. Soon your boobs won't feel full ever because your body is figuring out how much milk baby needs. Feeling empty has nothing to do with being empty. My boobs always feel empty (baby is four months) but she still gets plenty of milk out of them (sometimes I can see it around the edge of her mouth while she nurses, and she's gaining weight). "Empty feeling" boobs mean nothing. Just let baby keep nursing until baby quits, and then consider offering the other side.