Well, sometimes even if you have a little bit of data left from your previous data pack, this data is not enough to continue your running data session. For example, suppose you have 2MB remaining from your previous data pack, but that amount is not enough to browse the current website. In that case, to give you a seamless internet experience without any interruptions, you will be charged from pay as you go.
If you are annoyed by this, don’t worry, just turn your pay as you go off from the settings option in the app. Click here to know how to turn pay as you go off.