This may be caused by the recurring transaction of an invoice. Or someone that has access to your QuickBooks company made the transactions.
You'll need to check if there's an invoice recurring transaction. Locate the invoice that was sent, if you can see that, you'll have to Pause it before deleting it. This is how it looks like:
However, if there's no active recurring invoice that's having the same details as the sent ones. You should check the Audit log in the Settings. To know more about how to use this feature, you may check out this article: Use the audit log in QuickBooks Online.