It also tends to affect the neck and shoulder. Its cause is not entirely clear, though it may be related to eczema. This is what it sounds like your baby has, given the yellow crusts that you describe on the eyebrows. It is not caused by allergy to what she is eating, and the breastfeeding is not contributing to it. While it will go away by itself eventually, it can be treated with certain types of steroid-based creams, if necessary.
Infant eczema is also very common.
It usually starts between 1 and 5 months of age. In eczema, you have red bumps and patches in various locations. Eczema often affects both cheeks, on the face only, but it can be much more diffuse, and some babies will have the rash all over as they get older. While it's true that some babies with bad eczema have allergies such as to milk, in the majority of infants a specific cause cannot be found, other than a tendency to have dry skin. Some infants who have eczema outgrow it as they get older, while others continue to have problems with it throughout childhood.