My stomach fluid was reabsorbed before they could test the fluid in a recent 4 day hospital stay. should i be concerned it's cancer related?

Depends: This depends upon the type and quantity of fluid and the setting in which it was present. I am assuming this is fluid within the abdomen outside of the stomach. Example benign causes of fluid in the abdomen include: liver disease, infection/inflammation, and recent ovulation. This is a question best answered by your PCP. They can review all of the available information and help with any concerns.