Unsolicited calendar invitations

If running iOS 13 or earlier versions, check: Settings - Passwords & Accounts - Accounts - Any rogue entries here? If so, delete the rogue account. 

If running iOS 14, check: Settings - Calendar - Accounts - Any rogue entries here? If so, delete the rogue account. 

The Apple support article is below. 

https://support.apple.com/en-us/HT211076

You don't have any malware.

You allowed a website or app access to your calendar, they filled it with spam.

Delete spam calendars and events on iPhone - Apple Support