I would go see a lactation consultant ASAP. Try to find one who has one of the specialized scales that can measure "milk transfer" ie how much your dd is getting when she nurses. They may also be able to offer you weight checks, more frequently than you would go to the ped. Also watch her diapers. The key is to make sure she's hydrated and peeing enough, and gaining weight. If you think she's not getting enough, she's probably not. When I was pregnant, my dd was younger than yours, and I ignored my mama instincts telling me she wasn't getting enough, because everyone was saying she seemed happy, was meeting developmental milestones. She ended up loosing weight and refusing bottles. It is my greatest regret that I didn't see a lactation consultant. If supplementing becomes necessary, they will be able to help you find a way other than bottles to help keep your dd on the breast if you want to keep nursing her.