Trouble with vomit

[Benina]

My three-week-old DD is breastfeeding well but afterwards she often cries and looks miserable, and sometimes ends up being copiously sick. It started a week ago about once a day, but in the last couple of days she's been sick two or three times. The midwife and health visitor aren't worried because she's putting on lots of weight and seems well otherwise, but it's hard to see her getting so upset! We are burping her after each feed but it doesn't seem to help. Any ideas for what could be the problem?