Disappointed!!

I gave notice to cancel my policy with Oneplan on 29 May 2018 and received confirmation email that my policy has been cancelled. On 18 October 2019 i received an email from NuDebt collectors acting on behalf of OnePlan claiming that I owe them R268 and failure to pay will result in legal action against me. I had never been in arrears to the date of cancellation and fail to understand what I would possibly be owing them more than a year later. I had never been informed of outstanding amounts during cancellation or in the period between cancellation and handing over to debt collectors. My policy number with Oneplan was ***, I would like OnePlan to contact me with an explanation.