Someone else is sending transactional emails through my account

Hundreds of emails are being sent to people who aren’t in my contacts with weird messages. I have changed my password in Brevo, my email account and my website, I have also set up 2 factor identification with all of those. The emails are continuing to be sent. I cannot see what devices are being used with my Brevo account and need this to stop so my email address won’t be flagged as spam .