R4841.29 data account - NOT POSSIBLE!!!!

I've had this data contract for 21 months. For 20 of the 21 months the account has always been R299. One month it was just over R800. But this month, the account is R4841.29. HOW IS IT EVEN POSSIBLE??? The worst thing is that I only discovered that the account went over its CAP is because the contract stopped working. After calling MTN, they tell me it was because the account was \Over its limit"and that the amount was nearly R5000. The person told me that they couldn't say what happened as the account would only be closed off on the 20th. My current account is well over R6000 at this stage and no one can tell me what happened. I went to an MTN store where the lady tried her best to get through to someone at 808 and to log onto her Active account"