Why Is Your Weight Going Up & Down
Bowel Movements
Food takes a while to fully digest and goes through many stages, the last of which is through the large intestines which feces slowly passes through to be exited out the body. If you not going to the toliet regularly then this can build up which then increases your bodyweight and the inverse is also true.
Hydration Levels
There is a specific hormone in the body that regulates how much your body retains water and this can change by how regularly you drink water. If you drinking water irregularly and little of it then your body will do it's best to retain water. This is so it can it operate efficiently as a lot of biochemical reactions that happen in the body requires water. If you are drinking water regularly and in sufficient amount then less water retention will happen.
Carbohydrate Rich Diet
Carbohydrates are a great source of quick energy. When it is not needed it is stored in the liver and the muscles for later use. For every 1gram of carbohydrate that is stored, roughly 3-4g of water is stored with it. When you eat more carbohydrate food than usual then this is stored which increases water in the body which in turns increases bodyweight. Then when you engage in activity that raises your heart rate this storage is at least partly used up. It is also used up when you have not eaten in a while to provide your body with energy.
Salt
Salt plays an important role in the body from making muscles work and keeping fluid levels in check in the body through a complex system of monitoring and feedback loops. Fluctuations of salt intake through food will make you body either hold or release water.
Menstrual Cycle
The days before menstruation their will be a decrease in a certain hormone that will increase water retention, again increasing your bodyweight. This naturally decreases typically on the onset of menstruation period.