The scales thing is such a let down, the same thing happened to me a while back. I stood on my scales for weigh in day and thought i had lost heaps, stood on it again... got a different weight. I did it about 20 times, all different weights. Then when i switched to my other scales i found out i was heavier than i thought i was.
I always step on the scales about 3 times on weigh in day, just to make sure my scales aren't playing up. I'm thinking about buying myself a really good set of scales soon. If i'm going to be losing half my body weight, i guess it's a worthwhile investment lol. I think i might have broken my other ones from standing on them so much!
I agree with the other poster who said to take measurements, although i am really slack and never do it lol.
Hope all is well with you. Don't let the scales get you down! You are doing fantastic.