Sophie's started taking a lot less formula than usual. More often than not she will finish off 4oz every time but now she gets to about 2oz in the bottle and falls asleep! Other times she will finish off the 4oz and then crys for more milk and takes another 2oz. I'm wondering if shes just going through a growth spurt or she needs the next teat size up? At first when I give her the bottle she will just sit sucking on it for 5 minutes but isn't actually taking anything?