Vomiting could be a part of many illnesses in babies and children. It is not a major concern in general as long as the child is otherwise okay and free from other conditions. Vomiting is considered as common for infants, babies and children. The condition occurs when the food is brought back up from the stomach into the mouth and generally, the content is of stomach. All babies will throw up (vomit) a little. We call these spills or possets and these are the most usual cause of infant vomiting.
Sometimes, these possets are a little blood-stained - this is usually no cause for alarm. Milk coming from the stomach, back up the esophagus and out the mouth is also called gastro-esophageal reflux. Most of the time, vomiting in children is caused by gastroenteritis, usually due to a virus infecting the gastrointestinal tract. (Gastroenteritis is sometimes called the "stomach flu," which can also cause nausea and diarrhea.).
These infections usually don't last long and are more disruptive than dangerous. However, kids (especially infants) who are unable to take in enough fluids and also have diarrhea could become dehydrated. It's important to stay calm — vomiting is frightening for young children (and parents, too) and exhausting for kids of all ages. Offering plenty of reassurance to your child and taking measures to prevent dehydration are key for a quick recovery.
The most important factor in determining whether infant vomiting is a problem or not is to look at the rest of your baby. You don't have to worry if she is gaining weight, happy, developing normally (ie. learning new tricks all the time), otherwise well in herself.