you could try to figure out what was the cause of the mobile usage by going into your phone's setting and check which app used data. There could be a case when your phone is doing update but WiFi was cut off coincidently during that period.
if you do not have a pin number, contact CS_agent and provide them with the following to authenticate yourself. Email addressLast 4 digits from the credit/debit card on fileLast payment amount and datePublic Mobile account number