For most moms, drinking cow's milk does not pose any problem for their babies. While a baby cannot be allergic to her mother's milk, she can sometimes react to proteins in mom's diet. One of the most common proteins that babies can react to is the protein in cow's milk. So, while most babies do not have any problems or reactions when mom drinks cow's milk, there are a few babies who do react. It is only in these cases that mom should avoid drinking cow's milk and other dairy products.
Babies reacting to proteins in their mom's diet generally show the following types of symptoms:
Mucus/blood in stool
Excessive fussiness (usually in addition to one or both of the above symptoms)
These symptoms generally ease or disappear entirely within a few days of mom eliminating the offending food, though keep in mind that it can take a full two weeks to get dairy entirely out of your system.
There can be other causes for these symptoms as well, so if you notice any of these in your baby, it's best to have her examined by a doctor to rule out any other causes.