The key thing is that it began when you stopped b-feeding. I have had several friends that when they have eliminated dairy from their diet (either child or mother) it cleared right up. If your formula is milk based, you most likely will need to switch formulas. Also, you will need to supplement her with a probiotic since she has been on some antibiotics and this kills the good bacteria in their system making her digestive system (where 80% of the immune system is located) unable to fight the eczema. And she is not getting the enzymes she needs from your breastmilk no more. Whenever anyone takes a round of antibiotics, they need to replenish their immune system with a probiotic. I gave my daughter FloraBaby by ReNew Life, can be found at www.smartbomb.com
When I had to supplement I used Nutragimen since the milk based formula's would trigger gastric distress for her. What you eat comes out in your skin and dairy is a big cause of inflammation in the body for most people. So even though she is on formula, it will still trigger an inflammatory response in her system which is coming through in her skin.
For my daughter, since I was already dairy and gluten free, it was peanut butter and all nut butters that triggered the eczema while I was breastfeeding. The other thing is to avoid putting mineral based oils on her skin. Try avocado, coconut or olive oil on her skin, it is the best all natural product out there and to nourish it with good fats. Olive oil works for cradle cap too. A few other things, do try to switch laundry detergents like the other post said, I use Method Brand by Target and I also use organic cotton sheets which can be reasonably found at Babies R' Us, not stylish, but nice to know your baby is sleeping on something "green"
Make sure you find a derm that specializes in food allergies too, they are out there but hard to find. The book, "Is This Your Child?" By Dr. Doris Rapp has a section on eczema and food allergies, that is how I figured nuts and tree nuts. www.drrapp.com is her website. You can find the book on amazon too and I highly recommend it for your bookshelf.
Good Luck!