Sodium - Increasing the sodium in your diet temporarily can make a temporary water retention weight gain.
Time of Month - many women retain 1-8 lbs extra water around TOM
Exercise - yes exercise. The process of exercise and increased strength involves microtears of the muscle. Water is the transport system of the body to bring rebuilding materials to your muscles, and to flush toxins and waste created by the process away (e.g. lactic acid). The more fit you are in general the less fluctuation you will see from exercise. But any change in routine, new exercise can cause this shift. (I have personally gained 5 lbs in one day from exercise)
Shifts in the carb/protein ratio in your diet can effect water retention temporarily...basically ANY change in diet can change it one way or another.
Shifts in diet or exercise can effect the speed of your digestive system. Fluctuations can be cause by waste material that hasnt yet passed.
Humidity. You can gain and lose A LOT of water when you breathe. On a dry or air conditioned night you may lose more water weight and be more dehydrated than on a humid night.
Fluctations happen. That is why the PROCESS of losing weight is key. And realize that not every pound can be traced exactly to what you did yesterday. I did NOT lose weight this morning because I had dessert last night
Learn what fluctuations are normal for you and focus on your behaviors. If the process is there, then over time the scale will go down.