Excessive cell phone/data a/c unexplained

Autopage could not fix Blackberry after repeated attempts-had to buy another phone to tide me over. They finally agreed to upgrade a month early in November with new contract for end February-got Samsung S5 and bought extra data to cover me until then. I did not use my phone straightaway-went from shop on a business trip. Sms the next morning informed me of around R680 data charges-queried it immediately. Got back to Jhb to the shop-showed them data usage (26kb) and asked for reversal of charges-no credit yet (paid via debit order so have no control of payment). Bill used to average at around R575, now around double with no additional usage. No-one can tell me why despite repeated attempts