I was also just about to say that she could have an allergy to something in the Prevacid.
It could be possible. Especially if you're using the solutabs (which I assume she is since I think that's the only way to get that medication for infants). They're flavored and they have strange binders in them and could possibly even have lactose. When I used to get name-brand Prevacid, it used to be pink as well which tells me there was some kind of dye in it. Now that I have generic Prevacid, it's white (but still cherry flavored). This is why I'd suggest seeing an allergist, they'll be able to give you some better advice.
All I really know is that red dye #40 (which is in Tylenol IIRC and many other medicines) can be very intolerable to many, many people and ESPECIALLY to babies. So if you're going to use a pain killer, get some dye-free.