There could be several reasons why your AWS RDS (Relational Database Service) costs might suddenly double. Here are some potential causes:

1. Increased instance size: If you have scaled up your RDS instance to a larger size (e.g., from a t3.micro to a t3.large), it will significantly increase the cost, as larger instances are more expensive.

2. Additional instances: If you have added more RDS instances to your setup, such as for high availability or read replicas, it will increase your overall costs.

3. Increased storage: If your database storage requirements have grown, and you have provisioned more storage for your RDS instance, it will result in higher costs.

4. Backup storage: RDS automatically takes backups of your database, and if you have enabled longer retention periods or have a large amount of data, the backup storage costs can add up.

5. Data transfer: If you have a significant amount of data transfer in and out of your RDS instance, especially if it involves cross-region data transfer, it can contribute to higher costs.

6. Provisioned IOPS: If you have enabled Provisioned IOPS (input/output operations per second) for your RDS instance to achieve consistent performance, it comes with an additional cost.

7. Multi-AZ deployment: If you have enabled Multi-AZ deployment for high availability, it essentially creates a standby replica of your RDS instance in another Availability Zone, doubling your instance costs.

8. Unused instances: If you have RDS instances running that are no longer being actively used but haven't been terminated, they will continue to incur costs.

9. Change in pricing: AWS may have made changes to their RDS pricing model, resulting in increased costs for certain instance types or regions.

10. Increased usage: If your application's usage has significantly increased, leading to higher CPU utilization, I/O operations, or network traffic on your RDS instance, it can result in increased costs.

To identify the specific reason for the sudden increase in your RDS costs, you can review your AWS Cost and Usage Reports, which provide detailed breakdowns of your AWS charges. Additionally, you can enable billing alerts to receive notifications when your costs exceed a certain threshold.