As a massage therapist I hear a lot about hip and sacrum pain. The pain can mean many different things but here is a pose to help relieve the discomfort: This pose can also help with any menstrual tension.
Sit on the floor in a comfortable position. If your back is rounded due to tight hamstrings, place a pillow beneath you until you can sit up straight and comfortably. Bend at the knees and bring the soles of the feet together. If your knees are too high, and you are feeling knee pain. place pillows underneath each bent leg to support them. Hold onto your legs at the ankle. Slowly begin to fold forward over your bent legs. Hold the pose for about 10 breaths after you have warmed up. When you are done with butterfly pose, feel free to straighten the legs as in staff pose to relengthen the hamstrings.