All of a sudden two days ago I started getting a sever stabbing pain in my left nipple. It comes and goes but very painful. What could this be?

Hello, Anon.

Thank you for your message.

A sharp, burning or stabbing pain in one breast that is constant or intermittent is probably not related to your menstrual cycle and may be noncyclical breast pain. This type of breast pain may be inside, beneath, or near the breast. When it occurs, this sharp breast pain feels like it is in one specific area or trigger zone. There are a number of causes of noncyclical breast pain, including oral contraceptive pills, hormone therapy, and other medications. In addition, large breasts may also be painful, especially if a woman is not wearing a proper, supportive bra. If the pain persists or you are concerned, see your doctor.

More breast pain information here:
http://www.medicalnewstoday.com/articles/263566.php

Helena