A typical tummy tuck will cause only minor changes in some aspects of your breasts though enlargement is not one of them. Most probably, you are retaining a significant amount of water which is bloating you resulting in enlargement of your breasts. This is even more accentuated because now your abdomen is much flatter than before the procedure. You can weigh yourself or have your doctor weigh you to determine if there is a substantial increase in your weight. I have had a few patients over the years who were compulsive water drinkers with some putting on 10 or more pounds within a matter of a few days after surgery.
You should consider notifying your plastic surgeon of your findings. Less likely would be cardiac related issues. In some situations, diuretics may even be indicated.