Question about Amazon RDS and unknown cost

I posted question about unknown cost that i have to pay a few days ago, and got an answer from the AWS support, but could not sort out the problem. My Bill details shows that the cost i need to pay occured at RDS, but i have no idea why the cost keeps increasing everyday. I followed the directions to see if there is an existing RDS in AWS console, but i cannot find any. The bill shows RDS at Asia Pacific (Seoul) Provisioned Storage caused $0.06 and RDS Backup Storage caued $0.01. Even, active regions were 13 yesterday but i found out that it is 16 now today. In Amazon RDS console at all regions, i cannot find any database instance. Instead, if i go to both paramter group and option group, i am able to find there is a default:mysql-8-0. Once i tried to delete this, it shows an error saying status code:400, invalid db parameter group state.

can anyone please help me?