OVERCHARGING ON EPIC 100 CONTRACT

I am having problems with your billing and since i upgraded my contract to an epic 100 which is supposed to be R189.00 monthly. My epic 100 contract was supposed to start effect from the end of October when my old contract expired. They said that i only will be billed for the new contract end October but to my surprise i was billed R362.00 and currently R324.00 is due for this month, I can't understand how it can double in a month when i didn't even use it , i only bought like 4 data bundles for R19 this month but to my surprise its going up to R324.00 and i can't figure out how is that possible. I,m even not fond of using this phone anymore i rather just pay my subscription for the month and put this sim card far away because its ripping my finances without even an explanation for what's going on from your customer care assistants who hang up the phone in my ear. I,m even regretting the day i signed up for this contract because these charges are so exorbitant and your agents wont even give me a better answer as to how does it double.