It might probably get auto renewed as add-ons are auto renewed after every month (as long as there is an active base pack).
Call CC and ask them on what date the add-on is expiring for the current month. Then call them back on the day before that expiry date and cancel the add-on.
Note that there is no real way for us to know on when an add-on gets renewed every month for each of us (it is not necessarily on the 1st of every month). This date might differ from the base pack recharge date as we might have activated the base pack on one date and the add-on on a different date of the month.