Well I think the doorbell may have been ringing in Real Life and you heard that in your Dream. This has happened to me to like with my alarm clock, the phone, radio, etc. And don't forget you always have time to do a reality check thats the difference between becoming Lucid or missing the oppurtunity. But at least you had a Lucid no matter for how long it lasted at least you had one. May you have many more to come.