Stomach flu isn't really a 'flu.' It's gastroenteritis and can be caused by bacteria, a virus, or a parasite which can spread quickly through contaminated food or water. Pay attention to daily hygiene and food preparation & handling. Main symptoms are watery diarrhea and vomiting. Other symptoms are headache, fever, chills, and abdominal pain.
There is no medicine that will kill the stomach flu virus (antibiotics only work against infections caused by bacteria which can destroy both good and bad bacteria).
The good news is that most cases of gastroenteritis go away on their own, especially if caused by a virus. Keep yourself hydrated esp. your are continuing to BF and rest well.