It's an illusion of perspective with respect to the design of her dress, which is very flat and forward facing. If it is the shadow on the left that you're looking at then that's because it's misleading as well. She's holding it in a very awkward way so that she could clap. Obviously there are much better ways for doing that, but if you look at hands and the facing of her palms you'll see what I mean.

I don't know why, but I'm very perceptive tonight. This is weird.

Edit: Forgot the rest...

Anyway, with the way she was holding that, the light that was coming mostly from the stage direction hits the purse and casts a fairly distinct shadow over her lap, adding up with the already present shadows of local ambient occlusion it looks like boobs when it isn't.