I was just like you --> LOVED cow's milk and anything made from it (yogurt, ice cream, ricotta cheese, cream cheese, butter, etc., etc.). My son was just like your daughter --> cow's milk really messed up his digestion/poop and gave him horrid diaper rash (actually not a traditional "diaper rash" but rather an allergy rash). And we later came to find out it also caused his poor sleep (he couldn't stay asleep for long, didn't get restful sleep) and his reddish/dry cheeks and other tiny little patches of dry skin on his wrists, torso, legs, etc. (uh, eczema caused by the milk).
So... even if you think, "oh, cow's milk doesn't bother me, and it didn't bother her while nursing" --> sorry to be the one to say: it CAN bother your baby, even if it doesn't bother you. :(
I never thought we'd be able to eliminate cow's milk products. I thought it would be too tough. I thought I would love it so much that I couldn't give it up. But you know what? IT WAS SO WORTH IT. My little guy was sooo much happier, healthier, and calmer -- and so were we, his parents!
So, if you want to rule out the idea of cow's milk causing your baby's discomfort, there's really ONLY ONE sure-fire way of doing this: eliminate all cow's milk products for 4-6 weeks (seems like a long time, but it isn't, you *can* do this) and watch to see if her poop and rash issues go away. Then, do what's called a "challenge" --> give her a glass of milk or two, watch her for 48-72 hours for a reaction (could be the poop and blisters/rash thing, or being super fussy/tantrumy, or sleeping poorly, or little patches of dry skin showing up, or really dark circles under the eyes - see list of allergic symptoms in the Dr. Sears link below). If no reaction to the challenge, that's a good sign and you can proceed to reintroduce cow's milk, continuing to watch for any signs of allergy during that first week or so. If no signs, great! If signs, back off again.
Honestly, we did this *as a family* because I thought it would be too hard to keep milk away from my son while my daughter and my husband ate whatever they wanted. We went dairy-free as a family, and that turned out to be *really* helpful to have that support while making the change. I thought I'd miss cow's milk too much, that our family wouldn't survive it! Now that I look back on it, I can't believe I dreaded it so much. Our family did *great* and we found out that, indeed, for our son cow's milk WAS A PROBLEM, and that he (and all of us, actually) do better without it, and we don't feel deprived.
We didn't replace cow's milk with soy products, either (with my son, he was also sensitive to soy, and I read in one research study where kids who are allergic to cow's milk are 50% more likely to be allergic to soy, so I avoided it just to keep things simple). Instead, here's what we use as our substitutes - sharing them in case they'd help you at all:
* instead of milk --> Living Harvest brand hemp milk or Pacific Foods rice milk (available at local grocer or health food store, orderable online), light coconut milk (sold in cans)
* instead of buttermilk --> tablespoon of acid (such as lemon or lime juice, or vinegar) plus milk sub (coconut, hemp, rice milks)
* instead of butter --> coconut oil, dairy-free buttery spread like Earth's Balance or margarine, or even Crisco Butter Flavored Shortening
* instead of ice cream --> diary-free sorbets of all flavors including coconut sorbet and Hagen Dazs chocolate sorbet
* dark chocolate --> there are many great, high quality dairy-free chocolates, just read the labels, but a staple around our house is the Ghirardelli Semi-Sweet chocolate chips
* instead of cheese --> anything fatty and salty, for example, mayonnaise or avocado w/ salt or guacamole or olive tapenade or pesto made without cheese (*seriously easy* to make at home if you can't find a dairy-free version in the store)
* I've not yet been able to find a sub for yogurt or cream cheese, though regular (full fat) coconut milk is really tasty in smoothies in place of yogurt
Hopefully that will get you started - just print your hidden dairy "cheat sheet" (see link below) and make a trip to the store without the kids so you can start reading labels, and stock up on some basic dairy-free stuff. We ended up eating a lot more simple & healthy WHOLE foods because there is *no label* and what you see is what you get, guaranteed to be dairy free -- for example, apples, bananas, pears, grapes, sweet potatos, broccoli, oranges, blueberries, celery, bell peppers, carrots, watermelon. ALL of these things are easy to get, fast to "prepare", healthy, and VERY dairy-free. :)
Here are some links to help you get started...
Food Sensitivities in Babies
http://www.kellymom.com/babyconcerns/food-sensitivity.html
Non-Dairy Sources of Calcium
http://www.ellenskitchen.com/faqs/calcium.html
Hidden Dairy "Cheat Sheet"
http://www.kellymom.com/store/freehandouts/hidden-dairy01...
Tracking Down Food Allergies
http://askdrsears.com/html/4/t041800.asp
Dairy-Free Recipes
http://www.paleofood.com/
Whatever the cause of your daughters rash, I hope you can find it quickly so that you both feel better soon! If you do decide to rule out whether cow's milk is a problem by doing a temporary elimination, I just want to encourage you that you *can* do this (you do have the intelligence and the will power), and best of luck!