Confused

I have a contract for airtime and data with vodacom, for July I paid R180 and now I just received an invoice saying that I must pay R468 for August, how do I go from 171 to 500.i am sorry but I won't pay this money it's either you cancel my contract or you check your billing system and this is not the first time this has happened ,I also have a contract with Mtn but have never encountered any problems with them,