Lately sometimes when I give my son a bottle, he'all drink it and sound like he's going in on the bottle guzzling it down. Then I take it from him to see how much he drank and its at the same line when i made it. I made 2 ounces and he drink for about 10 minutes and it'll still be 2 ounces...what the heck is he doing and then act like he's full...