It's a Small pea size bump under the skin one week after breast implant- what is it?

Thank you for your question.  The most likely explanation is a syndrome called Mondors syndrome which is a small blood clot in a vessel.  It is generally self-limited and not dangerous however it is important that you consult a plastic surgeon who performed your breast augmentation.

There are many possibilities here ranging from completely benign and self-limited to bad things.  Best to see your surgeon who can evaluate it and appreciate where it is and follow it up to make sure it isn't getting larger or need to be removed/biopsied.