My six months old baby is pooping after every feed. and sometimes she also vomit. what could be the cause?

Hard to say: There is a normal reflex called the gastrocolic reflex where a signal to the intestines to move stuff along/poop occurs anytime they feed. Breast fed babies can poop every feed or skip days. There may also be other digestive issues. Vomiting is a non-specific issue that can range from semi-normal regurgitation (first yr) to a real illness. If it bothers you discuss it with her doc.