iPhone : Iphone and kindle

[Knipples]

Ok, so i've got my kindle and my iphone, and the iphone has the kindle app on it.

I've got it so I can sync between the two, i.e. I can read on my kindle, then go to work, want to carry on reading the book on my phone, and then it sync's it to the phone, and back and forth and so on.

Does anyone know if me reading my books on the phone is using up my data allowance, I know it would cost a bit of my data when it actually syncs, but is it continuing to use up data (if im not on wifi) when I have the app open and am reading the book? (as in until I close the app)