I don't know. I think perhaps she finally realized she's not the center of attention anymore? But seriously, she still looks great, should have stuck with the theme, but.....maybe something got in the way of personal life and she's just not into it. We, women, do this when personal matters happens.....
Heidi is gorgeous whatever she wears, but this isn't a night out at an Off-Broadway experimental play. She made herself look conspicuous by how underdressed she is. It's literally made you guess she's having 'a moment'. Many came dressed as though going to a church wedding, but she deliberately put on her least impressive pretty dress. I can hear her say (as she often did) a la Project Runway to a contestant "Where's the sexy in that?".
I think @bag-princess is right. If something personal was afoot, she should have phoned in sick. This is not a wedding or a friends birthday, she didn't have to go.