No matter its duration, menstrual bleeding is a secure sign that a woman is not pregnant. The question now is: did you recently experience a menstrual period or were you perhaps experiencing another type of vaginal bleeding (spotting)? If your previous periods last for the same duration (3 days), then this bleeding might also a period. If, however, your previous periods lasted longer than this bleeding, it may not be a period at all. In any case, you can take a home pregnancy test to be certain. And you can seek help from a gynecologist.