you most likely have endometriosis and will need either medication to reduce the cyst until it disappears or you will need surgery to remove the cyst.
http://www.gynaecologyclinic.com/endometriosis/endometriosis.htm
The pouch of douglas is the area between your rectum and the back wall of your uterus. Cystic means your mass looks fluid filled. Most masses of this type are benign. As devblue stated above...these types of lesions are most often associated with endometriosis. I am sure you doctor will help advise you of the best treatment.
Take care,
Shelly