It sounds like you do have a blood clot in your lungs as the lung ventilation/perfusion scan showed an abnormality. This is dangerous and potentially life threatening because it means that you could experience subsequent clots that completely block off blood supply to your lungs. Being on coumadin is the appropriate treatment and the duration of this will need to be determined by your doctor depending on the cause of the blood clot. By the way, the clots often start in the legs but are not always found on the ultrasound studies of the legs (which is why your doctors aren't sure if it started in your legs). The risks of coumadin include increased bleeding risk, but the benefits in your case by far outweight the risks.
Here is a link with more information: http://www.uptodate.com/patients/content/topic.do?topicKey=~GpplS8wqq66Q3G
Please let me know if you have more questions.
Thank you for responding so fast to my question. If I had not gone to the hospital because I did not feel well, could I have just dropped dead? Also, if they never find where the clot came from, how can more clots be prevented in the future? I think I will only be on coumadin for 8-12 weeks and then another scan will be taken. What will happen if it still shows an abnormality?
Yes, pulmonary emboli are potentially fatal. The way to prevent future clots is to treat with coumadin for a certain amount of time depending on whether the cause is reversible or not. If the repeat scan shows abnormality, you will likely need to stay on coumadin for longer.