My Top-up contract amount goes up every month

Every month my Top-up contract's amount goes up. However since this is a top up contract the monthly subscription can't go up as I took out the contract nearly 2 years ago and I was informed that this amount of R199 is fixed for 2 years. Last month my phone was suspended cause I was R40 short on my payment according to them although I did make the right payment. I paid R50 and my phone was unblocked. I spoke to 3 different consultants in this call to find out why I'm suspended and all 3 consultants had a different amount to what my monthly subscription is, which is very suspicious to me. I'm also technically R10 in credit with Cell C as I paid more than what I'm suppose too. This month I paid more towards my account to make sure that the subscription and the Get More service is paid for (as the Get more service where one of the consultants excuses why I was in arrears the previous month). Today I dial to buy myself some MB's and I get an error message, I phone Cell C and to my surprise I'm informed I'm R1.69 in arrears and that's the reason why I cant buy MB's, until this is paid. The R10 credit however has disappeared????!!!!!