how could I know if my phone's been hacked Someone is telling me every single item of information that I sent via whatsapp

how could I know if it is my phone or only the whatsapp account has been hacked

I use Samsong Galaxy S4 Gt-I9500
android 5.0.1
Check for the whatsapp web feature , from menu select "whatsapp web" , probably they got it authorized in your absence . And they can continue to access unless you log out or cookies are cleared.

you can logout from mobile app itself. choose "log out from all computers"

from home screen of whatsapp , click on the "more" menu "(three dots)" in the top right corner of screen. The drop down menu appears, there select "whatsapp web",
then there is list of sessions running, just choose "logout from all computers" under that list.