When the lining of the uterus sheds, it has to pass through the cervix so it can leave the body. The uterus contracts (squeezes) to do this, pushing the menstrual fluid out. When these contractions become painful we call them cramps. Not all women have cramps. It is common for girls who have just started their periods to have no pain, for instance. Women who have had babies, too, often report that even though they used to get cramps, they no longer do. But most women experience at least a little discomfort during their periods sometime or another.
Cramps tend to come with heavy bleeding, so they usually only last for the first day or two of your period, when there is the most bleeding. Sometimes they will begin before you start bleeding.
Cramps can be different in each woman.
They might be just tiny aches and twinges in her lower belly which are hardly noticeable. Or she might feel like a big hand is in her belly, squeezing hard. Or she might feel strange shooting pains that go up the insides of her thighs. Her lower back might hurt. Or she might just feel a certain kind of fullness inside - like a pressure that needs to be released.
I don't know. I don't think anyone knows for sure. We know cramps are usually come with heavy bleeding and the contractions (squeezing motions) of the uterus that cause this bleeding. The contractions should not have to hurt, however. Your intestines, for instance, contract to move food through them when you are digesting, and you usually cannot feel those contractions.
One theory is that cramps might be caused by having too much prostaglandin in your system. Prostaglandins are chemicals made by your body to help it do different things. Certain types of prostaglandin make your uterus contract.
But if there is too much prostaglandin being produced, the contractions of the uterus, which are supposed to be painless, become very strong and last longer than they should. As a result, oxygen can't get to the tightened muscles, and we feel this lack of oxygen as cramping (the same thing that happens when you get cramps from running).
The prostaglandin can also leak through the walls of your intestines and cause diarrhea, which some women get when they have cramps.
Isn't that nice to know?
( Note: severe cramping can be a symptom of a chronic disease called endometriosis)