The new iOS 9 has a new feature that will allow your iphone to switch between your data plan and wifi to keep a strong uninterrupted signal. This feature is on by default. Keep an eye on this! It can be very useful but some social media apps and other websites can pull some major bandwidth quickly! This is OK on wifi but if your iphone switches automatically between the two you may get a very high unwelcomed data bill!
Please look for a more detailed report here: