Are you using the same scales? I weighed myself using the scales at Publix (grocery store if you're not familiar with it...they usually have big mechanical scales in them) a week ago and I was the same weight I was 4 weeks ago...so it kind of worried me, and then I weighed myself yesterday at a different Publix and I had gained 6 pounds. I think it's more likely the scales were different.
Also home scales are usually really unreliable...and even some scales they have at the Dr's are unreliable. I always weighed 4 pounds less at my previous OB's office than anywhere else.