Crying baby... Help

Julia • Kiwi girl, Kindergarten Teacher and Aunty to one beautiful nephew. Expecting our first little one January 2015
My little 3.2week old has suddenly started screaming durring every feed. I think it's gas but I'm not sure as im a first time mum. She screams and pulls off durring feeding, vomiting up her food and is only happy when lying on her back. I can't seem to calm her. Any suggestions or advice??