What Causes Lower Abdominal Pain During Or Before Having Periods?

Hi..my name is pawan...my wife (age 21+) always suffer from severe pain in her lower abdomen during or just before her periods. And she used to take a medicine called Meftal Spas for last two years. My question is that..why she is having so severe pain at the time of her peroids, is this is natural or there might be some internal problem?..and my second questiin is that..wheather this MeftalSpas is suitable for her..is there is any side effects are also there..??