Allergic to BM?

Hey ladies! I had my baby girl 4/19. From the jump I was breast feeding and formula feeding her. It was a 75/25 split so maybe formula once a day. She was good with that till she was about 13 days old. All of sudden baby girl started throwing up after every feeding and the diarrhea was so bad her diaper couldn’t hold it. The on call doc didn’t seem too concerned Bc she wasn’t having a fever. So I just started giving her little portions at a time. An ounce at most till she poops or throws up before giving her another ounce. THis just meant that she was throwing up little portions at a time cause it didn’t stop the vomiting.

This weekend I decided to do a full day of breast milk no formula and another day of formula no breast milk to see the issue. Well, she only throws up when I mix or give her breast milk. My diet hasn’t changed since I’ve gotten home. I’m eating almost the exact same thing every day (cultural practice) so I’m trying to figure out why after 13 days of breastfeeding she all of a sudden can’t hold breast milk down? Is this normal? She’s gaining about an ounce a day which is okay Bc she was born tiny. So weight gain isn’t being impacted here. Help??