What Causes Excessive Bleeding During Menstruation?

My 22 year old daughter is having an extremely heavy period with lots of clotting. She is sexually active but has not ever been pregnant. She was complaining of being weak & dizzy & for 2 days she was very pale colored with little color even to her lips. Could she bleed too much & that be why she was weak, dizzy, & pale. Her color is back now & bleeding has slowed down but still passing some clots. This all happened over the holidays & during her time of being very pale we were unable to get in to see the doctor. Should I still take her to the doctor or no since the bleeding has slowed & her cir is back??