Hi,
Welcome to the ehealthforum.
A small amount of fluid in the Pouch of Douglas can be a normal finding following the rupture of a follicle and release of an egg at the time of ovulation. Other possibility is of an abscess in the pelvic cavity which can be due to an infection and which may need antibiotics or drainage.
I suggest you to discuss it with a gynecologist and get a CT scan done.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.