If you are going to weigh daily or not, it is about the overall trend, not the day to day. Like at school -- it's your overall GPA over time that lets you graduate. One single grade from a pop quiz freshman year isn't going to doom your whole school career just because it wasn't want you wanted to get.
Pee, poo, food being digested, water -- it all has to weigh something inside you while processing. It doesn't mean the weight "gain" you see is fat tissue.
Put rocks in your pocket -- a gain. Take them out -- a loss. It is the same thing. Scales tell you weight change. They do not tell you what it is made of.
In one day I range almost 3 lbs. Less when I first wake up, more by the time I go to bed. And I know 3 lbs will disappear by the next day.
Try not to get emotionally involved in the scale results and try to take it calmly. Throwing in the towel because of one scale reading is premature. It isn't going to doom your entire weight loss/fitness journey.
Hang in there!
A.