It would be nice to see the window that was showing your Fidelity password.
Did it appear after you had entered it to sign in? It could very well have come directly from them. It may have been a note saying they need to have a new password because of their own system. Some financial sites require changing your passwords at regular intervals. Sites get hacked very often as AT&T has finally admitted (although they were asked by police to hold up reporting during investigations). Not sure what deleting the password you just created will do.
It (and most other) passwords are probably still in Keychain. Hopefully, you can look through its display and find it when you need to log in to Fidelity.
Unless you have the rarest of security apps known to man, I seriously doubt your Keychain or computer has been attached. The vast majority of us are rarely the subject of an malware attack. Very few malware apps will even run on a Mac. And even then, if you are using a reliable password app, all that might be "harvested" will be an encrypted mess of characters, not the actual passwords themselves.
I have never used Keychain because of its clunky and non-intuitive interface. It is the poorest "Apple-designed" app I've ever seen. However, that does not mean I don't secure my hundreds of passwords! If you feel your home is safe at least keep a copy of your passwords in a less-than-likely-to-be-guesed place. What I actually use and highly recommend is 1Password.
Other security steps include never using Chrome-based browsers.
