Your menstrual cycle is your body’s way of preparing for a possible pregnancy each month. Understanding how the process works is important. You can use this information to better manage any menstrual symptoms you are experiencing, and understand when there might be a problem. 
What is menstruation? 

Menstruation is the technical term for getting your period. About once a month, women who have gone through puberty will experience menstrual bleeding. This happens because the lining of the uterus has prepared itself for a possible pregnancy by becoming thicker and richer in blood vessels. If pregnancy does not occur, this thickened lining is shed, accompanied by bleeding. Bleeding usually lasts for 3-8 days. For most women, menstruation happens in a fairly regular, predictable pattern. The length of time from the first day of one period to the first day of the next period normally ranges from 21-35 days. 

How does the menstrual cycle work? 

The menstrual cycle is controlled by a complex orchestra of hormones, produced by two structures in the brain, the pituitary gland and the hypothalamus along with the ovaries. The menstrual cycle includes several phases.

General overview of the menstrual cycle: 

Days 1-5: The first day of menstrual bleeding is considered Day 1 of the cycle. Your period can last anywhere from 3 to 8 days, but 5 days is average. Bleeding is usually heaviest on the first 2 days. 

Days 6-14: Once the bleeding stops, the uterine lining (also called the endometrium) begins to prepare for the possibility of a pregnancy. The uterine lining becomes thicker and enriched in blood and nutrients. 

Day 14-25: Somewhere around day 14, an egg is released from one of the ovaries and begins its journey down the fallopian tubes to the uterus. If sperm are present in the fallopian tube at this time, fertilization can occur. In this case the fertilized egg will travel to the uterus and attempt to implant in the uterine wall. 

Days 25-28: If the egg was not fertilized or implantation does not occur, hormonal changes signal the uterus to prepare to shed its lining, and the egg breaks down and is shed along with lining. The cycle begins again on Day 1 menstrual bleeding. 

Why do I feel pain during my menstural cycle? 

Menstrual cramps are most likely caused by an excess of prostaglandis, a hormone that releases from the uterine lining as it prepares to be shed. Prostaglandins helps the uterus contract and relax. This is a necessary part of the process, but in excess, causes pain if the uterus contracts too strongly, blood flow is reduced, and the supply of oxygen to the uterus muscle tissue decreases, causing pain. For most people with period cramps, it’s still unknown what predisposes them, and not others, to painful menstruation.