Gf is almost half my weight and this is so embarring?

I think your blowing it all out of context, it shouldn't matter what she weighs. Funny how you say your embarred yet your underweight yourself, can hardly judge others. Women are hardly often supposed to be near their guys weight. She most likely lied because she was worried for this kind of reaction, she most likely doesn't want you having a go for being underweight. She needs your support most likely, not a go at her.