Do you have any menstrual disturbance ?
, lower abdominal pain
, vaginal discharge
and pain on having sex ?
Usually free fluid in Pouch of Douglas could indicate possibility of infection of your genitourinary system or abdomen. Due to gravity, pus and other infective material can get collected. In such cases you will have fever, lower abdominal pain and sometimes vaginal discharge.
Sometimes blood can also get collected there. This can be seen in a condition called Endometriosis
where the menstrual tissue may be seen inside abdomen also.
Usually it is not very significant unless you have above mentioned symptoms.
Kindly follow up with your Doctor and discuss the possibilities of above mentioned conditions. The cause for this free fluid should be found and treated.
Wishing good health.