Look in Settings->Filters and Settings->Forwarding. Is there anything defined?
Do you access your account with an e-mail client (Outlook, Thunderbird, Pocomail, etc), phone, notifier, or any other add-on/tool/web-site/etc? Is anything enabled in Settings->POP/IMAP?
Check Settings -> Accounts and Import -> Change Account Settings -> Other Google account settings -> Authorizing applications & sites. Have you given account access to any other web-sites or utilities?