The biggest short term impact on weight is water retention due to sodium levels. If I go out to dinner or eat sushi (generally accompanied with soya sauce) I can easily gain 3-4 lbs overnight. How quickly it comes off depends on the time of year. In the summer I can sweat out the extra sodium in one ride. In the winter it might come down by 1/2 to 1 lb/day. You might want to look at your eating patterns.
I recently started running and noticed that sore legs also seem to come with extra weight that goes away.