Are you weighing yourself at the same time every day? Our weight can fluctuate by as much as 5 lbs a day due to the amount of water we drank, ate, what we processed through our bodies etc. My doctor recommended weighing in the morning, as that's the best representation.
Also if you're on Atkins and slipped off it and starting consuming carbs - your body will gain water weight since it needs more water to process those carbs (at least that's how my doctor explained it to me)