Why is my AWS RDS (postgres) at a high percentage of cpu with no active sessions?
I can't guarantee this will always work, but what managed to be effective for me was switching to a different instance, then switching back. 

To do so, go up to Modify button in the RDS &gt; Databases &gt; [MY_DB} view, and then change your instance to any other instance type, wait for the update, then switch back.

I suppose I just had a bad ec2.mirco, and the method above got me a different (better) one which does not exhaust more cpu credits at idle than it gains.