Extreme pain and numbness in left arm following breast augmentation. Is it nerve damage?

While discomfort is not uncommon after breast augmentation, uncontrolled extreme pain is not. I encourage you to contact your surgeon to discuss your symptoms. He/She is most familiar with the details of your surgery and how you've been progressing through recovery.

Although some (often asymmetric) discomfort after breast augmentation surgery is to be expected,  what you are describing seems somewhat extreme. Therefore, best to follow up with your plastic surgeon (earlier that already scheduled, if necessary) for an in-person evaluation;  online consultants can only speculate. Best wishes.

My advice is that you immediately inform your PS about the issue just to be on the safe side.  Nevertheless, pain and numbness are common after breast augmentation that could last for a couple of days.  Ask your doctor for medications that can help you cope with the discomfort. But as your body recovers and your tissue adjusts to the new contour, the pain and other post-op symptoms are supposed to subside.  Oftentimes, patients feel okay a week or two after their surgery. Good luck and best wishes.