is this fluid get collected in the area within the first 6 hours ? if so this is likely to be blood otherwise I do feel from the way you've said it is likely, part of the inflammatory process and healing secondary to contusion
Now , it depends of how big of this collection ? How close it is from the skin ? is it deep or superficial? This will require further studies to find out about these concerns, I am expecting that you have seen some body (previously) to role out fractures
and wounds in the involved area.
The last thing is should there any constitutional symptoms ( fevers, chills
and shivering ) this needed to be attended by a specialist . This might indicate infection required immediate drainage
with antibiotica... and so..on.