Swift - self.navigationController is nil

I have a strange issue with navigation stack.

Overview: I have three view controllers (e.g AViewController, BViewController, CViewController) with two navigation possibilities:

1. AViewController to CViewController.
2. AViewController to BViewController to CViewController, or

And AViewController is embedded in a UINavigationController. I am using segues for transitions, all defined in storyboard. And all segues are of Show (e.g. Push) type. And all the transitions are performed using performSegueWithIdentifier(String, AnyObject?) method.

Problem: With the first transition, everything works fine. Everything means self.navigationController.popViewController(animated) in CViewController works fine, to go back to AViewController. But with second transition, self.navigationController is nil in CViewController, although it is not nil in BViewController.

So where am I getting wrong ?