Sudden High CPU Usage On RDS Postgres
Hi It's not about just not the number of number of session or queries which are running.

Below will help you to understand how to troubleshoot this problem


Download the logs from RDS console for this instance to generate the Pgbader report. Below is the format which is most suitable to generate the logs
Download the pgbadger and use below command to generate the report.
https://github.com/darold/pgbadger
Match your CLoudwatch or RDS CPU time stamp with Pgbagder report to see how many queries are getting executed 
Try to find the query which may be eating a lot of CPU.
pgbadger -q /postgresql*.log -f stderr -b "2019-07-17 08:55:19" -e "2019-07-22 07:30:07" -o pgbadger.html


I may be able to give specific guidance if you can hare more data.

Cheers,
Aj
http://www.postgresql-blog.com