Problem again after upgrade

It seems I have this problem every single time I upgrade. I took out a contract (a cheaper contract) with more benefits for the purpose of saving money. Before the upgrade my account was about R500 every month. Last month I got one for R700 (I assumed this was due to additional fees so I left it) then this month my account is over R900. My calls are R0 my sms's are like R2 and my DATA cost are over R400 and a Promo Service Fee of R113? My Subscription fee is only R254 a month how is it possible that my account is R906? It seems like the R400 occured when my pacage went over but I also had additional data on my old contract so why am I know being charged. If MTN does not sort this out I am moving to CellC (I assume you know of their promotion).