My balance is dropping even though I'm not running anything. Why?

The usual cause is storage you've forgotten about. Block volumes bill for as long as they exist, attached or not, and deleting an instance does not delete its volumes. A separately provisioned Shared Filesystem also bills on its own. Check Storage → Block volumes (and shared filesystems) in the console and delete anything you no longer need - turning on Remind when storage is unattached under Billing → Billing notifications flags this automatically going forward. If the charges still don't add up, contact support for a usage report.