I'd try not to fall out over it. I don't think either of you are in the wrong - you have a right to feel a bit upset at being 'pushed out', but I think she has a right to feel comfortable in her own home.
Perhaps just say that you'll have to only visit for a short time next time, as you weren't happy feeding alone, but you don't want to make her feel uncomfortable.
Another thing, do you know why she bottle feeds? Perhaps she wanted to and wasn't able to, and watching someone feed successfully might make her a bit upset? I know that doesn't make it easier for you, but it could explain it. I know I'd feel quite emotional seeing someone feeding - I'm exclusively expressing as we haven't managed breastfeeding yet.