I had shortness of breath after c section, but lung study negative. Why didn't they wait for that before starting thinner?

PE? First i'd say ask your md, instead of strangers online. Second, i assume you mean the "study" was to look for pulmonary embolism. If you have a pe, and we wait for the study to prove it before treating, you could die. The risk of bleeding from a blood thinner is often less than the risk of untreated pe. Medicine is simply weighing risks and benefits, but the catch is defining them precisely.