Could it be that the initial weight loss was a great deal of retained water? I've been an on & off low-carb dieter for several years (I'm prone to giving in to temptations, sometimes

... ) and the first week, I always lose the most.
Instead of potatoes, try baked spaghetti squash. They're 8g of digestable carbs ber 1 cup serving, but well worth it. You can dress them up with cheddar and bacon and scallions and you can't tell the difference between it and a baked potato!