Why my account was charged using RDS?

Please check RDS free tier page.

Are you using db.t2.micro? It is the only DB with the free tier.

Additionally, you need to

• use SQL Server Express Edition.
• have max DB storage: 20 GB of general purpose (SSD)
• have max 20 GB of storage for database backups and database snapshots

It is free for 750 hours per month up to 12 months.

With different configurations for SQL Server, you will be charged.

Shortcut version of free tier requirements is here.

There isn't enough information in the question to have a guess.

I find the best thing to do when querying unknown costs is to use the "Bills" section of the Billing Dashboard. AWS bills monthly (including current incomplete month) and each bill beaks down by Service, Region and Component which should make it easy to track down & identify exactly what is generating the $50 charge.

AWS also splits bills by Account when using AWS Organisations to manage multiple environments/clients/units/etc.