Incorrect billing

Why is my data cost so high. I've struggled for the past two weeks to load data bundles. I phoned the call centre and your agent also struggled. She was supposed to call me on Monday. I have a call limit and I know that if you don't have bundles the costs will be deducted from the airtime. So why is my bill over 3 grand when there's a call limit. Why am I being billed on the new contract when the sms' that you've sent state that it'll be effective 1 Nov. if I'm being billed on it, shouldn't it be pro-rated