It's usually perfectly normal for women to have one breast a little bigger/smaller than the other breast.
As far as post delivery, are you breastfeeding? I ask because I've read that one breast can produce more milk than the other.
Now if you've had quite a considerable shrinkage in one or it's changing position, I would definately see a doctor just to be on the safe side.
It may be due to the milk in your breasts after delivery. One side of your body is always bigger, also. Us humans aren't perfectly symmetrical.