What could free fluid in an ankle after a severe inversion injury mean?

Ankle inversion: Resting while elevating the injured ankle above heart level, compression such as with an ace wrap or preferably a more secure ankle splint, and ice usually help. Time is the limiting factor in most cases. Aspiration may be tried in severe cases but most of the fluid will return unless due to traumatic bleeding into the joint.

Swelling: The only thing the body can do to protect itself is swell. It is an indicator of the presence of injury, but unfortunately does not hint at severity or specific problem, that is up to exam and imaging (usually mri) with interpretation by good orthopedist of foot/ankle specialist, like podiatrist.