Issue with iOS7 and UIPageViewController - ios

Question

I have a UIPageViewController that is added as subview on UIView. The UIView sits on the top of UIViewController's view, at start it puts the UIPageViewController lower than it should sit, but right after I click on the next button to push the next UIPageViewController's view, it send it back to the correct position.
What is he doing that? I tried everything but still can figure out the problem.
This is how it looks:
What seems to be the problem? Thanks in advance!
EDIT: This is my code:
- (void)viewDidLoad
{
[super viewDidLoad];
// Do any additional setup after loading the view from its nib.
self.pageController = [[UIPageViewController alloc] initWithTransitionStyle:UIPageViewControllerTransitionStyleScroll navigationOrientation:UIPageViewControllerNavigationOrientationHorizontal options:nil];
DataChildViewController *initialViewController = [self viewControllerAtIndex:0];
NSArray *viewControllers = [NSArray arrayWithObject:initialViewController];
[self.pageController setViewControllers:viewControllers direction:UIPageViewControllerNavigationDirectionForward animated:NO completion:nil];
self.pageController.dataSource = self;
self.pageController.delegate = self;
self.pageController.view.userInteractionEnabled = NO;
self.pageController.view.frame = self.view.bounds;
[self.pageControllerParent addSubview:self.pageController.view];
[self addChildViewController:self.pageController];
self.currentPageControllerIndex = 1;
}
- (IBAction)buttonNextDidClicked
{
self.buttonNext.enabled = NO;
__weak typeof(self) __self = self;
[self.pageController setViewControllers:#[[self viewControllerAtIndex:self.currentPageControllerIndex]]
direction:UIPageViewControllerNavigationDirectionForward
animated:YES
completion:^(BOOL finished) {
__self.currentPageControllerIndex++;
__self.buttonNext.enabled = YES;
}];
}
#pragma mark - UIPageViewControllerDelegate & DataSource
- (DataChildViewController *)viewControllerAtIndex:(NSUInteger)index
{
DataChildViewController *childViewController = [[DataChildViewController alloc] initWithNibName:#"DataChildViewController" bundle:nil];
childViewController.index = index;
return childViewController;
}

Answer 1

So, I've figured out the problem.
On viewDidLoad I had this line:
self.pageController.view.frame = self.view.bounds;
When it actually should be:
self.pageController.view.frame = self.pageControllerParent.bounds;