why my domain falls?

I'd take a look at your CPU utilization metrics on the instance's "Metrics" page. Do metrics show that CPU utilization metrics are consistently in the burstable zone during times of increased traffic to your instance? If so, this could be indicative that you need a larger instance. This doc article may help with understanding metrics around CPU utilization, and when to move to a larger bundle size: https://lightsail.aws.amazon.com/ls/docs/en_us/articles/amazon-lightsail-viewing-instance-health-metrics#cpu-utilization-zones.