Q.R.
C.,
I've found that happens when the baby is wanting more milk to come out of the bottle than is coming. Usually for bottles like Avent, they have sizes for the nipples on the bottle, and you start with a #1 then move up to a #2, etc. as they grow. I don't know so much about DR Browns bottles, but maybe you should try moving to a bottle or a nipple that puts out more milk, or poke some more holes in it.:)lol. Hopefully that helps. I just know my son had the same problem. He was starving until I changed the nipple on the bottle and he did much better. It just sounds like he is getting frusterated that he is not able to get very much milk out fast enough anymore. Lots of luck!
Qandryel