Occasionally, a cyst which is growing on a stalk from an ovary may twist the stalk on itself (a torsion).

http://www.patient.co.uk/health/ovarian-cyst-leaflet

• Occasionally, a cyst which is growing on a stalk from an ovary may twist the stalk on itself (a torsion). This stops the blood flowing through the stalk to the cyst and causes the cyst to lose its blood supply. This can cause sudden severe pain in your lower abdomen.