Lately I have noticed that Lily squirms a lot when eating.  She pulls away from the bottle and starts crying.  She gets stiff and arches her back and turns into a wail. I can't figure out what her problem is.  I thought at first it might be that we tried to move up to the stage two nipples and she wasn't ready.  So I switched back to the stage one to see if that makes a difference.  It doesn't seem to be gas either.
Re: Q: what is this behavior?
This is right around when Aerin's reflux developed. We had to go to Dr. Brown's bottles stage 1 nipples and keep her upright for a while after her bottle. Also, we had to do less in a bottle, with more frequent feedings.