Contract length extended after cancellation being requested and processed

I have already processed my contract cancellation, wanting to go over to prepaid, on 2 March 2018. I was informed that my contract began on 30/4/2016 and my day of billing is the first of the month, which meant that my first bill would go off on 1 June 2016. I signed a contract for 24 months, which meant the effective date of expiry would be 31/3/2018 and my last bill would go off on 1 May 2018.

I was told that because I received airtime on 1 May 2018, that will need to pay for the period and be billed on 1 June 2018. But I did not request that my contract airtime continue in May.

This effectively means that my contract was for 25 months and not 24 months, even though I requested the contract cancellation 2 months before the expected last billing date.