Incorrect billing resulting in extremely high subscription

Since 2007/ 8 l have had a contract with cell c. Network was great. Billing great. Great partnership. Things started changing in July 2017, where by my contract subscription more than doubled. Until today l have been quietly paying, but today l decided it's ridiculous l cannot be paying this much. I use work Wi-Fi during the day first of all. And l am mostly on whatsapp, emails and Facebook and a few different sites, even then it's not for long periods. On top of my normal package l usually top up with a 2GB or 3GB to help me through the month also a whatsapp bundle. What baffles me is the fact that l get charged over R2000 a month which is said to be for data usage. My whole thing is if l am topping up why do l have out of bundle charges whilst l have an active data bundle. Simone tried to assist me as much as she could but the feedback she gave me has been unsatisfactory to me as she could not explain to me what the charges are for. It is totally ridiculous. I need Cell C to fix this ASAP. I have this same issue with two of my accounts. I am holding on to a contract that does not help me much because Cell C coverage in Mthatha sucks so much that l use a second phone which is an extra cost for me as well. I am told that this 2000 was used over 4 days. And to my knowledge l had an active bundle at that point. I would like an urgent response. All this is messing up with my budget. Regards