Hacked

See these security-related documents, especially the first two.

If you think your Apple ID has been compromised https://support.apple.com/en-us/HT204145

Security and your Apple ID https://support.apple.com/en-us/HT201303

iCloud security overview  https://support.apple.com/en-us/HT202303

Ways to keep your information safe on Mac https://support.apple.com/en-gb/guide/mac-help/mh11402/mac

Using app-specific passwords https://support.apple.com/en-us/HT204397

UseTwo-factor authentication for Apple ID https://support.apple.com/en-us/HT204915

You can also do a backup and try a factory reset. Then restore from your backup. When adding back downloaded app, check each one carefully. If you still have problems, do another reset and don't restore from your backup.

Restore to Factory setting.      

Backup - Restore your device from an iCloud or iTunes backup.