I am a 24yr old female and for the last 3days my left nipple has been hurting me quite bad and is still hurting me what could it be ?

Nipple Pain: Breast and nipple pain sometimes occurs as a result of hormone changes during your cycle, even if that never occurred before. Typically, these involve both breasts. One sided pain, however, should be followed up by your gynecologist, just to be sure there isn't an underlying infection or other cause of the symptoms sometimes a small benign lesion can obstruct the terminal lacticiferous duct.