My infant has a milky discharge coming from her nipples. Is this common, or should I be concerned?

Common. This is a common finding in young breastfed infants, both male and female. It is not a cause for concern unless the nipple appears red or tender, or if the baby is irritable or appears ill.
Usually normal.... ..Unless it persists longer than about 2 months (from birth), or if the breast is red and swollen and tender.
It's common. The hormone (prolactin) that causes you to secrete breast milk passes into your baby through your milk. Both boys and girls may get small, firm lumps under the nipples, and often will express breast milk, especially when the nipples are lightly squeezed. This is completely normal and not a cause for concern. This can persist for months, especially in breast fed babies.