There are many different causes to consider, and an important clue iin diagnosis s how long your dog actually keeps her food down. If she is throwing up the entire meal, that may indicate eating too quickly, or that she's developed a food allergy. If you feed once a day, split that amount up into two separate meals. Check the labels on the food for grains. Corn is a big offender in dog allergies and is very common in the cheaper grocery store foods.
If this problem has been going on for more than two weeks, that warrants a visit to your vet. She may have picked up an infection and needs a quick round of antibiotics.
sometimes it might be from eating too quickly or too much.
i would agree with shara21.
try to ge her to eat a little more slowly if you could.
does she eat very quickly?
Does she throw up right after eating (undigested food) Or a while after eating?