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In brief: Water. Free fluid is typically water. The lining of your abdominal contents are slippery (your bowel, the lining of your organs) and it's thus wet inside your abdomen. It's normal to have a little free fluid in one's pelvis, as that's where fluid tends to pool.
Anechoic (black) areas on ultrasound indicate fluid. In your case, free fluid just means water that was seen in that area was not encapsuled. A small amount of free fluid in the post CDS is physiologicially normal,especially in a person that is well hydrated and drinks plenty of water..