What Does Presence Of Fluid In The Pouch Of Douglas Indicate?

I have just had a pelvic ultrasound and they have found I have fluid in the pouch of douglas? What exactly does this mean? I am due to have another ultrasound in a few weeks to check again. 3 months ago I miscarried naturally. I then went on to have my first period and bleed for 6 weeks until the doctor put me on the pill to regulate my hormones. What could fluid in the pouch of douglas be from?