4 month old spitting up after nursing, but not after bottle feeding....

[Morgasaursmom]

My DS is 4 months old, and spits up after he nurses, every time.  He does not spit up though, after he drinks from a bottle.  He only gets breastmilk, never any formula.  He has spit up since birth, was diagnosed with reflux, and we have found that Prevacid keeps his discomfort under control.  He used to spit up ALL the time, but it now mostly occurs just after nursing.  He has had ok weight gain, dr wants to see more.  He gained 2lbs in 2 months, is now 26in long, weighs 13lbs.  He was 9lb 4oz at birth.  His height percentile is around 85%, but his weight percentile dropped to 16%.  He is healthy, no other issues.  I'm just baffled as to why he only spits up after nursing and not from a bottle....thoughts???

[CookAMH]

Can it be simply because of baby's position? Laying down more to nurse and sitting up more to take a bottle?