Menstrual periods are the bleeding that results from the monthly changes in hormone levels. During the first half of the menstrual cycle, during bleeding and the weeks after that, the dominant hormone is estrogen. After ovulation, the domininant hormone is progesterone. Progesterone causes the lining of the uterus to mature. It also causes changes in special muscle tissue in the body called smooth muscle. These muscle are in the intestines and some women notice slowed intestinal action, constipation and gas in the weeks right before a period.
These muscles are also in the walls of the arterial blood vessels. Typically these muscles contract to keep our blood evenly distributed throughout our body even though we change position or move rapidly. Think about a hose full of water. If you suddenly lift the hose, the water all flows downward and spills out. Blood in our vessels wants to flow downward the same as any other liquid. This would cause less blood flowing to the brain and result in a light headed or dizzy sensation. By contracting when we move, the smooth muscle in the arteries keeps the blood moving upward with us preventing these symptoms. Then progesterone enters the scene and slows this reaction, sometimes allowing a brief decrease in flow to the brain. Until their circulatory system adjusts, some women will then experience brief dizziness or lightheadedness during the progesterone dominant part of their menstrual cycle.