Hi, welcome to the forums.
I assume you only use webmail to check emails, and not an email client/app such as Thunderbird, claws mail etc?
If so - it is totally recommended that you use something at least as a back up in case of situations such as this.
I don't believe it's anything like being hacked, otherwise unread emails would probably not show either.
You say it's BTMail, is your account BTYahoo or BTMail? If it's BTYahoo, then there is a Yahoo image at the top.
I've not checked, but is there an option to hide all read emails?