Hacked

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. 

No, your iPhone has not been hacked, but it is subject to calendar SPAM if you are not careful on what sites you visit.

Ref: Remove spam from Calendar on your iPhone - Apple Support