Brief Answer: causes are many Detailed Answer: The Pouch of Douglas is the deepest point in the peritoneal cavity. It is a collection point of fluids during the monthly ovulation and when there is an infection in the peritoneal cavity. Fluid in POD (Pouch of Douglas) can be due to blood, pus or other fluids. The causes could vary from : ovulation irritable bowel syndrome and reproductive concerns like ectopic pregnancy rupture of ovarian cysts Pelvic inflammatory disease (PID) The exact cause can only be made out after careful evaluation and examination of the fluid along with correlation with your symptoms and examination fining. Since free fluid is accompanied with back pain you should do a check up to rule out any pelvic infection. If infective cause is detected, and if you are sexually active, get your partner also treated simultaneously to prevent chances of re-infection, and avoid having intercourse while being on treatment. Take care of your self

---