Billed after number has been converted to prepaid

I cancelled my contracts (2) end of January and the consultant informed me that the last debit order will be end of February, and the number will then be moved to prepaid. She informed me that I will receive airtime on the 1st of March, when would I like to convert to prepaid, if I convert immediately on the 1st I will lose the airtime received on contract. I asked if I wait until end of March until data and all airtime is finished will I get charged extra, she said no, last debit is end of February 2016 since payment is made in advance. so I said lets convert it end of March them, by then I would have finished all the benefits. She said she has processed everything; I even got a text message confirming that. End of March I got an sms that my numbers are now prepaid, then I got a debit order sms that cell c has debited money.