You just lost a lot of water weight that first week and you are now readjusting. I did Atkins once a long time ago (10 plus years ago). It worked for a while. I imagine it would have kept on working if I had not gotten bored of it. Just restricting calories and exercising seems to work better for me. The good thing about Atkins is that really it restricts calories. Eat the hamburger, but don't eat the bun. You have now restricted your calories. I understand the mechanics behind it: little carbs, body needs to use fat stores. Either way, don't be discouraged. Your body is confused. Make sure you weigh yourself at the same time each week and only once a week (I actually weigh myself more often but only go off of my weigh in day weight), and decrease sodium intake which will increase water retention. I would also say to track everything that you eat which is hopefully what you are doing here. Keep up the good work.
Goal weight 117 by 2014