Why do some women have severe pains when mentruating?

Why do some women have severe pains when mentruating?
There can be many reasons. Endometriosis, Polycystic Ovaries, anatomic abnormalities....the list is very long. If you are having severe pain, see a gynocologist.
It is the woman's uterine muscles cramping
it depends upon their body health. if the uterus is week then there is the chance. some ladies have very small vaginal path, so that time also pain will come. one more reason is heredity.
the bigger the clot the more pain it takes to pass it out of the uterus. try taking some aspirin this will help thin the blood making the clots smaller and less painful. im on my period now and was in severe pain which woke me up! two asprins later i feel better x