What causes body weight to go up or down?
So what exactly is moving the scale in the upwards direction? It's hard to pinpoint exactly what a particular person's body is responding to, but the majority of weight fluctuations can be attributed to water weight and things like hydration, exercise, bathroom habits, and what you have eaten that day. Other areas to consider include: high intake of carbohydrates, salt intake, hormonal fluctuations in women, and constipation can all cause the scale to show a higher number.
Just like the scale can bump up quickly, it can also drop, if for example, you have been sweating at the gym and did not hydrate while exercising. Try jumping on the scale after a hardcore workout and you may be surprised that your weight is down.
Get off and hydrate and eat some carbohydrates and the number will probably be right back up to where it was before you worked out. And no, you didn't just lose and gain a few pounds of actual fat in a couple of hours.