Stomach flu

Stephanie • My names Stephanie, I`m 25, a wife and mom to two kids, and expecting #3!
How long is typical for a baby to have the stomach flu? Two people at my in laws had it and it lasted a day, maybe two. My poor baby got it and has had diarrhea and projectile vomiting for a week now. I've taken him to both the ER and doctor who say he's fine and send him home, but after a week I'm starting to feel helpless for him(and am questioning if it's even the stomach flu).
I called the nurse again and she didn't ask anything, just said it's persistent and no need to bring him in. 
I don't want to be that over worrying mother who keeps bringing their baby in but this is happening after every bottle and I'm really starting to worry 😭 any advice is appreciated