excessive data charge

i have a 2gig data package as from november at R179.00 per month including in this package i received a vodacom powertab10, which is the total package. this month i received an account of R1800.00 for data usage which i beleive is incorrect and i need an explanation why this contract is now an open contract (has somebody changed my contract un beknown to me) as i always take out top up contracts, all my previous mothly costs were correct at R179.00. Thanks Gert Cloete