Huge amount in billing all of a sudden

It all started when our phone went to be repaired (was still under guarantee), when it came back, the data was finishing after a few days. When we complained, CellC could not find a reason, told us to buy data. We rep**** to them that we had the contract for 10 months already, it never happened before and our usage has not changed. Not having any answers for us, they then told us we were put on the wrong plan, that we must change the plan, which we agreed on, hoping it would resolve the problem. Last month our bill was more than double the amount we have been paying so far. When we contacted CellC, they told us that it was the result of the change in the plan (if it was charges-related we were never told about them, the change was supposed to bring down the bill not up!)... which is not convincing at all, because for the first 10 months or so of the contract (since June 2014) our bill was stable and our usage the same......they cannot answer us in this regard, .....we do not know whom to go to to resolve this matter and seriously thinking of moving to another service provider once the contract is finished and discourage friends and family to go to CellC.....