My girl used to drink 90ml milk every 3 hours. Then at about 6-7week, she can finish 120ml of milk. But lately (about 2months old) she hasn't been able to finish her milk, sometimes drink 90ml or even less. And she has been able to sleep longer, so she can sleep for 4hours without waking up and hungry for milk. I'm worried that she isn't drinking enough. Should I be concerned?