Debit order canceled but still got debited

I called rain this month early after they sent out messages that their prices would be going up and canceled my service with them, the agent assured me that I won't be debited this month end, I asked why I am not getting anything by mail to confirm my cancelation, she said that it will be sent on the last day of this month, I want to know why I was debited when I clearly indicated that I don't want it anymore, the internet has been very slow and found myself restarting my phone just to get a connection more than 5times a day