FTM looking for advice/encouragement

My 8 day old had her first checkup today.. the doctor acted concerned about her weight. She was born at 37w5d and weighed 7 pounds (but she was also very swollen and had fluid in her belly). When I left the hospital she was 6.6 and today she was 6.9.. up until today she has been doing amazing at BF but today it seems like she just wants to sleep and barely eats.. and of course it’s on a day where I am feeling discouraged and want her to remind me that we are doing okay and we should continue breastfeeding. Any ideas as to why she’s suddenly acting more sleepy than usual? Still have plenty of wet and poopy diapers..