She realized herself that a fake Naveen existed when she first talked to frog Naveen (She said something like: "If you're the prince, who's waltzing with Lottie?"). However, when she saw the fake Naveen marrying Lottie, she was very dumb for not realizing he was the fake one. Also, she was aware that the real Naveen never talked to Charlotte, it would be very weird if they arranged the marriage in 20 minutes (that was approximately the time while she last talked to Naveen and she saw the marriage).

Well she didn't know that Naveen changed/loved her so I think she would've thought that Naveen really ditched her for Lottie. Plus I think her emotions just kicked in when she saw Naveen and Lottie together, without the logic that there was a fake Naveen.

But according to mama odie, they both would become human again after the kiss. So the fact that Tiana was still a frog should have given her a clue, right?

I could see it both ways, but I'm surprised she didn't think of it. Oh well that's Disney, they do love a good story with some drama here and there.

I had forgotten about it! I guess she probably thought that he was transformed into a frog after walzing with Lotie... I don't know, I have a lot of issues about that movie anyway.

She did know Naveen loved her- Ray told her Naveen said it. And I never thought about it before, but this is yet ANOTHER loophole in the movie. This movie is just so terrible.

@Swanpride Your point doesn't make sense, because fake Naveen and Charlotted didn't really kiss before Tiana saw then. Anyway, another failure in the movie, since Tiana didn't think of this.

She was heart broken and couldn't think straight, I guess.

Tsk tsk.