Hi L.,
I was going to actually suggest she had reflux and then you said she's on Zantac already.
I would call your doctor. I don't believe the Zantac is working for her... if there really seems to be no connection to anything for her screams than it is probably something on the inside. The diarrhea could be relative to digesting too much stomach acid, or it could be related to your breastmilk.
My son would get like this when his dosage needed to be adjusted due to weight gain. I can't speak from experience with colic, but I had a reflux baby and it sounds like that is flaring up and isn't controlled with the meds.
Technically, it's not too early to be teething, but it is pretty unlikely for the majority of babies. Did your 3 yr old get teeth this early? I've found all my kids got theirs early but they were each consistently early. By 1st birthday they all had 16 teeth in their mouths :)
Talk with your doctor about her medicine. See if there is another she can recommend. Some babies end up on two meds because they work differently ( some work by preventing acid from being created, reducing acid that's already present and some by reducing the acidity of the stomach contents so that if it does splash up it's not going to be as painful and damaging).
Good luck to you... reflux babies are challenging in ways different than typical babies :) just remember to keep your patience... she's only doing it because it hurts :(