My DD started doing that at about the same age, we resorted to feeding her in her car seat, bouncy chair, or even the high chair in a tilted position so she wouldn't be flat on her back, then making sure she didn't fall asleep on the bottle. Every feed we would try in our arms first and then resort to other means. Eventually she stopped on her own. I'm not sure if we did the "right" thing, or if it would even work for you, but we did this when it got desperate.