Hello Anurag Srivastava,
Thanks for your question.
The billing model charges for resources from the time they are created till they are deleted. So, even if you canceled the API Management service within a few hours, charges are gotten for the period it was active. See billing model here:
https://azure.microsoft.com/en-us/pricing/details/api-management/
You can view exactly what caused the charges from Azure cost management > Cost analysis on the Azure Portal.
If you view the above and see any discrepances, I would recommend reaching out to billing support. This service is completely free and they would be able to assist you. Create a ticket here:
https://azure.microsoft.com/en-us/support/create-ticket
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola