Weighing less at night?

My RNY was on 12/30/11 and I feel like I'm making "alright" progress on my weight loss however for the last 4 nights I have been weighing between 243 and 244.5 when I get up the next morning I weigh between 245.5-246.5. I'm on the pureed stage my daily calorie intake is between 300-400 I'm getting my water in..but still struggling a bit with protein..I've been getting up to about 40-45g a day. So I guess my real question is..does anyone know why I'm weighing less at night? And why have I been stuck at the 245ish range for 4 days anyways? Thanks so much! 

You could be retaining water. I know I sometimes wake up and my ring is tighter than it is during the day and it is directly related to water retention.