Overcharged

I have a contract phone that i took out last year in July. It is a fixed payment and i pay about R 159 per month. During the month of September I paid R 541 and in October the bill seems to be on R 995. they keep on saying that i have bought data using the self service option but i dont even know what that is. What I usually do is I buy airtime and convert it into data once my bundles have been depleted. I use my own money to buy airtime and convert and i use *141*2#. The next thing mtn tells me that I they also charged me from their side for buying the data. This actually means that mtn is double charging me. All the data that they are referring to is the one i buy myself with MY OWN AIRTIME. How do they bill me for something like that. I receive airtime on the 14th of every month, but recently they have been sending airtime and a gigabyte bundle which is coming from MTN. I never signed up for that and i don't understand because that's what they bill me for. I have a feeling that there is something wrong here and there is a mis-communication somewhere