I have Mirena in and my counselor said that is probably what is plateauing me. I was consistently 3 lbs/week up until 3 weeks ago. Since then I've lost 1.8, 1.4, and .6 (in order) and I've been cramping (no period) on and off that whole time.
i haven't had a period in 1 yr (since Mirena was inserted) and still haven't on this diet, but I feel like I am PMSing 3days/week for the last 3 weeks.
Counselor said the estrogen being released from fat so quickly is 'excess' since I'm getting enough with the IUD, which is causing my PMS symptoms AND stalling my weight loss.
I haven't cheated or done anything else different from the previous weeks so I'm really bummed. I'm within 8 lbs of my goal so I made the decision yesterday to go to phase 2. There is still the possibility to lose and I know (personally) I just can't do another week of phase 1 and lose less than 1.5 lbs.

I also have had VERY low labito (TMI) which struck me as odd since I feel great and am so much more comfortable with my body. But when you research "excess estrogen" that is also one of the top side effects. So I'm pretty confident in this diagnosis.
Wish me luck on the next phases as I am REALLY hoping to lose at least another 5 so I can stay in the 120s in maintenance. I don't ever want to see 140s again!!! (and hopefully not 130s either)