Based on your description looks like you upgraded after expiration, and if this is the case any unused credit expires, and your subscription transitions to standard Pay-As-You-Go rates
You will only be charged for costs accumulated within the pay as you go subscription.
When you sign up for an Azure free account, you receive a credit valid for 30 days. If you upgrade to a Pay-As-You-Go subscription before the 30-day period ends, any remaining credit remains available until the end of that period.
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Regards,
Abiola