The issue isn't antacids per say. Maybe. The Doctors don't even know, as they told you.
But your baby is so young.
And, perhaps from increasing her dosage... without knowing how much to do it by and IF you can do that, it caused a reaction.
If her antacid, was seemingly ineffective... then tell the Pediatrician. Before increasing dosage.
In a newborn, their organs/kidneys are not yet fully developed nor their nervous system. But as the Doctor said, there is no way to singly attribute her reaction, to the antacid.
For 'antacids' our Pediatrician said that only to give Maalox.
And to NEVER give a baby or child, that "pink" liquid stuff. (sorry I forget the name of it) because it is not for babies/young children and causes some kind of problem.
Anyway, I'm not a doctor.
But, maybe just look up Zantac online and its side effects. And look up, side effects of Antacids in newborns, in general.