CDS likely refers to the pelvic Cul De Sac. The pelvic cul de sac is the lower area of the pelvis or abdomen (the area framed by the lower portions of your pelvic bones.)
There is normally a small amount of fluid in this area. In the event of infections, ruptures, or conditions such as cysts or cancer, the amount of fluid can increase. That the fluid is free simply means it is not inside a mass, cyst, or organ.
I would interpret the finding, "small free fluid" to possibly indicate the normal condition.
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