The allergy was not caused by your intake. Your child probably had a predisposition to the allergy already, and the exposure through the breast milk just brought it out in her sooner. Almost definitely, she would have had the allergy anywhere.
The question of whether exposure through breast milk affects allergies or not is HIGHLY debated amongst allergists, docs, and researchers right now. No one knows for sure whether or not it actually affects anything. Some docs will tell you to avoid dairy altogether, while other docs will insist that breast feeding is the best way to prevent against allergy. Most will admit that they don't know for sure and that admit that half the docs will say the opposite of whatever your own doc says.
All that said, now that you know she has the allergy, eliminating dairy from your own diet WILL make a difference. However, it will only matter if you eliminate it 100% and not just reduce it. This means no milk, yogurt, cheese, ice cream, or any product at all that contains milk, whey, or casein. You have to learn to read labels very carefully, as dairy ingredients appear in a lot of places you would not expect them to.
I breastfed my son exclusively (no formula ever) for just over a year and consumed dairy every single day. Aside from mild eczema, we had NO IDEA that he a dairy allergy. He can't eat even a single goldfish cracker without having a major reaction. The docs just don't know...
K.