There are three common rashes that babies can have in the first few months of life. Baby acne is very common and usually starts by about 3 weeks of age. In baby acne, you see red bumps that look like pimples. The bumps are on the cheeks, forehead, and temples. They do not cause any discomfort to the baby, though it often can look very distressing to parents. No treatment is needed for infant acne. It occurs because of the hormonal changes that are occurring in the baby after birth, and will go away on its own in several weeks. You don't need to put any special creams or lotions on it, and you definitely want to avoid any acne products made for teenagers and adults.
Another common rash at this age is seborrhea, also known as seborrheic dermatitis. It usually starts by one month of age. When you begin to notice red bumps (usually smaller than the pimples of acne) and yellow crusty scales and flakes on the hair-bearing areas of the body, such as the eyebrows, eyelashes, and scalp. 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. Eczema is treated with moisturizing agents, and if it's severe, may also need to be treated with steroid creams or other prescription ointments.
Mention the rash to your baby's doctor at her next visit. If it doesn't seem to be disappearing on its own, or it seems to worsen, I reccomend that you have the rash checked.