Menstrual cramps a few days before the period starts is common. Three/four days prior to the menstrual period many women feel cramps, yet have no menstrual bleeding. Cramps before the menstrual period is the activity of large and tiny uterine blood vessels.
Usually four days before the menstrual period uterine blood vessels begin to tighten, reduce, and pull down. As blood vessels tighten they reduce nutrients and oxygen to uterine lining. The reduction in nutrients and oxygen creates a lack of support to the uterine lining. As a result the uterine lining begins to break down which then results in menstrual fluid and such.
This blood vessel activity is why many women experience cramps a few days before the actual menstrual period. Large myometrial blood vessel shut down first. These larger blood vessels can cause strong cramps three/four days before the menstrual period starts. After the large blood vessel shut down, tiny blood vessels pull/shut down. Women often feel this a milder cramps/spasms.
For those who experience moderate to strong menstrual cramps, it is beneficial to take ibuprofen (store brand is fine) as soon as you feel pre-menstrual cramps.