Progesterone is released which further increases the blood vessels in the uterus. The egg is released during ovulation. If the egg is not fertilized the levels of progesterone and estrogen drop and menstruation occurs. The blood vessels are shed from the uterus during menstruation, the first day usually being the heaviest. Menstrual cramps usually occur on the first day of the menstruation cycle when the flow begins.
Cramps can last for a couple of hours to a few days.
The cramps feel like muscles spasms or contractions in the lower abdomen. They can be felt in the lower back and thighs too. These cramps can be mild or very painful. In some women it varies from month to month. In other women, the cramps are always very painful.