I have arm and hand numbness after Breast Augmentation, is this common?

This is a very good question.  The most important thing is that you have already updated your Plastic Surgeon and he/she is aware that this is happening.  They are correct in saying that your numbness is likely due to positioning of your arms during the surgery.  If the arms and hands are not padded or positioned correctly, there can be compression of the nerves in the arms and hands.  It sounds like this may have happened, but remember I cannot examine you so I cannot make a diagnosis.  It is likely that it will all resolve on its own with some time, but you must stay vigilant and watch for your improvement and make sure your surgeon is always updated on your recovery.  If you do not start to see improvement you may need to be seen by a Hand and Upper Extremity Surgeon.  Best of luck!