What Happend to shae?

In the beginning we meet her and and she is this smart, loving and funny character and then ur telling me she is to stupid to understand or accept that it's to dangerous for her? And she just betrayed Tyrion like that??? I felt like that was very out of character, also I'm surprised in the end she was just a "whore" when she was introduced i felt like she was gonna be a secret princess or warrior or something more lol