I have a pain at the top of my windpipe, behind it. What could be causing this?

Foreign body/throat. This might be many things. It can be just irritation, or it can be asore throat from infection it might be pneumonia but there would be fever, etc. Another possibility is that it could be a foreign body that was accidently swallowed and is stuck somewhere in the lungs/windpipe. If this has been going on for a while, you should see your doctor for an xray to be sure nothing is there.
Pharyngitis. Due to swelling from inflammation or infection ... Could be due to something you swallowed that injured (scratched) or is still stuck in (e.g. Fish bone) your pharynx or esophagus ... Could be tumor ...