T3 Free Tier causes costs after 7 months, but nothing changed

I've created my amazon AWS account on 2022-08-28 for trying out the service under the free tier. I've created a t3.micro instance back then in Stockholm, because t3.micro instances were available there which are covered by the free tier.

I've been running this happily for the first 6 months. I was not charged during this time, as this is covered by the free tier.

However, on 2023-05-02 I received an EMail from amazon aws with the title "Amazon Web Services Billing Statement Available", wherein it states that my account will be charged $34.19. The amount was billed to my credit card and I see it on my bank statement.

This is a surprise to me. There should not be a bill. The billing information states that the billig amount is for $0.05 per vCPU-Hour of T3 CPU Credits, and that 574.569 vCPU-Hours were used, which amount to $ 28.73 before tax.

This must be wrong. The T3 has a feature which can cause it to use more vCPUs. This feature is probably switched on, but this feature cannot have been used, because the t3.micro instance runs only a single-threaded process since I started using it last year. It's the same process. The process wasn't restarted. The process does number crunching only. It uses only one thread.

The process has been running there for 6 months and I didn't get billed, and now suddenly amazon aws claims that the instance started to use more vCPUs. This is impossible.

There is something wrong with how amazon AWS counts CPU usage. How to object the billing and notify amazon AWS about their failure?