Since you posted this in the CAD forum, I figured I would answer you, even though I have never been on Atkins.

It is unusual for someone to gain that much weight after a week on CAD, but would depend on a lot of factors, such as what you were on before you started CAD. It would be really helpful for us to be able to know exactly what you ate, if you are looking for an explanation of why you gained weight. There are many hidden things that can mess CAD up if we aren't really careful.

No matter what you end up doing, good luck to you