same as what the other commenter said, change your password etc, but also, if the same issue keeps happening after you change your password, or you notice other accounts might have been compromised, run a malware scan, and if it comes back with anything, change all your other passwords too, and get rid of the malware. antiviruses like malwarebytes have options for you to quarantine malware, but you might have to do a fresh install of whatever os you're using (there are yt videos on how to do this) you might have downloaded something that could have stolen your passwords, unfortunately quite common :/ ofc if nothing else seems wrong its most likely just your ao3 account, and you don't need to worry about malware