so bummed

I'm so bummed... I thought my baby and I were doing really well breastfeeding until we had her one week check up today. She was 6.4 when we left the hospital, but today she was 5.8. Dr seemed concerned and said if she doesn't start gaining an oz a day we will have to supplement. I want to keep breast feeding so bad but I feel horrible that she's not eating enough. Sorry for the rant. Just feeling pretty defeated right now😔