The confusion happens because the two verbs share ground in similar situations, but the difference is way more subtle than it seems! "Estou cansada" is the state right now, that's true, but "fico cansada" isn't just about change - it's more about a situation that settles or holds for a while, you know? Like, "fico cansada quando trabalho muito" (it's something recurring, a tendency of yours) versus "estou cansada hoje porque não dormi" (a one-off state). "Ficar" has this weight of "end up in such a state" or "stay that way," while "estar" is more neutral, it just describes the situation as it is in that moment, without implying duration or process.