High cpu usage on RDS MySQL instance for about 4 hours today

There's a knowledge doc that addresses exactly this scenario https://repost.aws/knowledge-center/rds-sql-server-resolve-high-cpu-use You should definitely be looking at setting up gathering metrics with Performance Insights and CloudWatch (if you haven't already) in case this recurs.

Do you have backups setup for this database, and what time of day so they normally run? Could it be that it's normally an incremental (which you hardly notice) but maybe today it was a full?

Without knowing more about the RDS instance it's difficult to give more specific advice.

It is a db.t3.small, so I can't enable performance insights. The default automated backup is enabled to run at 06:15 UTC with 0.5hrs duration with 1 day retention. I am on PDT and when we experienced the high CPU usage was way past the 06:15UTC time.