30 lbs in a month is a big loss! I wouldn't let one week of no loss pull the plug on that progress. You might just be retaining water. Besides, I don't know of any eating plan that can guarantee a loss every week. Our bodies have to catch up sometimes and as long as you're averaging a good loss, keep on keeping on. 5-6 pounds a week is a lot to lose!
The first part of any diet can be bittersweet. You get great losses b/c of the extra water weight you lose and while that can pump you up, it can bring you down when there's no more water weight to lose and loss slows down. But, at this rate you should be all done with water weight and what you should be losing now is fat.
I know that medifast is stringent. It's like 960 calories a day and that's crazy low. What is your doc saying? Plans this low in calories should be overseen by a Doc to make sure it's not harming your body.
Calorie counting gave me a method to fix the madness.
A high protein, moderate carb balance gives me control over cravings.