Keep getting charged

Hi, I signed up for the business trial account just to test/try out the business features. I entered my debit card details. However, after using it only for a day, I realised that it is not really suitable for me, and I cancelled the trail account. However, I am still getting charged for it even though I've already cancelled the trial account. Fortunately, I used my prepaid debit card (which does not have much value in it until I topped up the card), which explains why the payment did not get through (due to insufficient funds). But I should not even get charged in the first place. What is happening? Can you pls investigate?