Vodacom strikes again!

My contract recently came to an end, BUT I decided to stay with Vodacom, unfortunately. On the month I switched from a expensive contract to a top up package, they decided that they were going to ***** me with a R4500 data bill. No one could explain to me how, when or why? I spent HOURS on the phone, mostly on hold between departments, to no avail. I went to Vodacom Fourways and even the guy helping me said \something was wrong"but after a phone call "