You sure you are the only person that has access to your account? You must remember that webmail services are vulnerable to attacks. I usually set some "honey traps" in my email by sending a clickbait type email to myself from another account and then I do not open it. (Example : My nude photos or Bank Statements) .... if your account is hacked, those people will never leave those types of email out of curiosity.
It might be that, or you simply forgot that you opened it and used it or it expired after 48 hours and not the 24 hours you think it had.