This is a common complaint after a reduction. Basically the down sizing and lifting of the breasts off of the upper abdomen 'unmasks' the tummy. In other words, you can finally see your tummy without your breasts in the way so you notice it more.
Thank you for the question. It is not unusual for patients who have undergone breast reduction surgery to start to notice other body parts, such as the abdominal wall, once the breasts are reduced/lifted. In other words, it is more likely that you are noticing your upper abdominal wall area now that the breasts have lifted off the abdominal wall. Best wishes.