Hello Alberto Caprini,
Thanks for your question.
It could be your Free trial credits were exhausted or a service not in scope of free was deployed. Azure also includes “Always Free” services, but not everything remains free. These are the Free services in question:
https://azure.microsoft.com/en-us/pricing/free-services
To check exactly what caused the billing:
Go to Azure Portal > Cost Management + Billing and Click on Invoices → View Charges to see exactly what you’re being billed for.
You can also reach out to billing for free here:
https://azure.microsoft.com/en-us/support/create-ticket
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola