Sharlie, I agree that only a true allergy can be tested, however often an intolerance will show up as occult (hidden, not visible) blood in the stool.
The frothy poop you describe sounds like it is something else she is ingesting. Does she get any formula, any water, meds of any kind (gas meds, tylenol)? My ds had very frothy poops after he started on prevacid. Prior to that they were disgusting - looked like someone had blown their nose in his diaper, long strings of brown/green mucus.
Some babies are intolerant to nuts/seeds. Some babies are intolerant to certain kinds of fruit, citrus fruit is the big one (pineapple, oranges, strawberries, etc - my MIL is allergic to citrus). I generally do not believe that babies will be gassy from gassy foods that we eat, although if you have an intolerance to something (lets say onions) then your immune system could produce something into your milk that may make her react.
With such a sensitive baby like this I encourage you to keep nursing!! Do not wean to formula ... it will be much worse than what you have now.
Check into a different reflux med if you think the reflux is still bothering her. There are ones that are administered differently, like prevacid. Zantac will not heal damage of the esophogas, it just changes the balance of acids in the tummy.
An lastly, if she does not seem upset by the poops and doesn't have a rash, then you don't need to be too concerned about the color/consistency at her age. Make sure to delay solids until at least 6 months with her sensitive tummy.