Hi, so my girlfriend is getting dark brown discharge about 2 days before she is expecting her period. what could this be?

Her period: That is probably just part of her period. The small amounts of dark brown discharge can occur before or after full period. Nothing to worry about unless she has a fever and/or severe pain.

Vaginal discharge: Would check a pregnancy test. If positive seek medical attention, within 24 hours, If negative pregnancy test and no abdominal pain you could wait till her period comes, if the period doesn't come seek medical attention. If there is pain with the discharge,even if the pregnancy test is negative I would seek urgent medical attention either in an urgent care, your doctor or an ER,

Brown discharge: If this happens every month, and there are no other concerning symptoms (no fever, no severe cramping), it may just be from menstrual blood that has been sitting in the uterus long enough to turn dark brown. As more of the uterine lining is shed over the next few days, the discharge increases in amount and appears the usual medium red color.

Normal: Brown discharge could be minimal slow spotting prior to the normal menstrual flow. If this occurs on a regular cycle the discharge could be normal. If no signs of vaginal irritation or odor then this may be normal.