Just wondering about some possible causes. How close to the subject are you holding the airbrush? Did the airbrush perform well just prior to this result? What thinner or medium did you use? Lastly, does the paint still appear the same when you are loading the airbrush, has it gotten thicker or seem to have reacted to something? My first guess, holding it too far from the subject.
Don's point above is exactly right, if the needle has been deformed, it could have a really negative impact on your spray pattern. Best of luck getting it resolved.
Patrick