You likely have some stray video frames out on your timeline.
Press the \ key on your keyboard (above your Enter key). This will show you your movie's entire timeline. Chances are that it's showing you about three minutes of blank area beyond your actual project.
Click and drag around your timeline, in the area beyond your project, then press Delete. Then press the \ key again. Keep doing this until you've cleaned up all the stray video frames your editing have left out beyond your project.
Thanks for your help Steve, greatly appreciated.
Glad that you found and deleted the "orphans." They can come from a few different sources. This ARTICLE will give you a bit of background, and also some other things to look for.
Good luck and happy editing,