Charged for something a year later

My account was recently debited for R150 from me and you mobile. I enquired what the debit was for a week a ago and no one was able to tell and said they will look in to it. I have called again today and now I am told that I was charged for data in June 2015!! Ever since I signed up with me and you mobile from when they started, I have a debit order every month from me and you mobile and my account is always paid up. I think is really ********* to charge for something a year later! You claim to be a service provider with no contracts and a month to month account. How is possible to charge me a year later. If this is not resolved I will definitely cancel mine and my family's me and you mobile accounts.