I did the exact same thing on phase I, but a smaller version....lost 6 lbs week one, and gained one lb the second week. It was sooo encouraging week 1, and then felt like a bubble burst! Just remember, this is not a miracle diet...most of your weight loss was water weight, and then get rid of cravings....it is a change in the way you eat. You will probably do what a typical diet does...which is lose 1/2 - 2 lbs/week.
I do think you need to make sure you get enough calories...I would often eat nuts in the evening to get me above 1000 calories.
Welcome!! Hope it works for you!!
Also - I personally wouldn't stay on phase I. You aren't going to drop 5 lbs a week, and it is very restrictive. Read the threads carefully on how to transition though, otherwise it is *really* easy to fall off the wagon.
And I swear I lost weight more easily by eating my yogurt every day!