Without very good testable supporting evidence for what you surely must admit is an extraordinary claim, there is no good reason to believe that you actually did see a dragon flying by your window. In the absence of such evidence, the most excruciatingly open-minded skeptic will simply withhold judgement on the matter. The rest of us, given how unlikely is the story, will decide that you are most probably mistaken or that you are making up a story.
"Why?", you ask. Nothing in our experience tells us that flying dragons have ever existed with humans (some dinosaurs might qualify, but they disappeared long before our species came along). On the other hand, we have plenty of experience with humans making up stories and it is well known that humans very often make mistakes in interpreting or processing the information they gather through their senses. (Scientific methods of investigation are designed specifically to compensate for such human frailties.)
So, it is reasonable to conclude that it is most probably true that you did not see a dragon flying by your window. Your subjective experience might be valid for you but rationally skeptical individuals are not likely to accept it as evidence that the event actually took place as described. If I saw something like that myself, I'd be more inclined to think that I'd hallucinated than that there actually was a dragon.
However, if someone were to actually produce said dragon, then one would have to reconsider.