From email address not changing per user
In addition to the emails shown in the Mailings menu » From Email Addresses, you can also use the email address associated with the user you're logged in as.  I would check Clare's record to see what emails are there; it sounds like your email might be present on Clare's record.

If you want to restrict users to ONLY use emails avilable in Mailings menu » From Email Addresses, you can go to Administer menu » System Settings » Outbound Mail and set Allow mail from logged in contact to No.  Note that in older versions of CiviCRM I believe this option is located somewhere in Administer menu » CiviMail.