Does it mean that the woman is wrong or that (for example) you picked the wrong woman or both? (maybe I'm not understanding well the sentence in English, Am I probably λανθασμένη?)
You picked the wrong woman. It's more common to say η λάθος γυναίκα, though. Λανθασμένος is used for something of wrong logic/ knowledge. Λανθασμένη απάντηση=wrong answer, λανθασμένη λογική=wrong logic, λανθασμένο επιχείρημα=wrong argument.
And of course λανθασμένος is wrong too (from λανθάνω), λαθεμένος (from λαθεύω) is the correct word to convey this meaning but it is almost obsolete now. But languages evolve etc. etc. :)