What Causes Pain In The Nipple?

Greetings, Im male aged 20, I have pain on my left nipple. When pressed, it feels more like a bruise. I have not been in any physical activity and there has been no occasion where the area has been hit. I have a history of getting the odd skin infection (boils) once or twice a year, but i'm quite sure this isn't one. The area of pain is no larger than the tip of an index finger, it is not inflamed and there is only the sensation of pressure, like from a bruise. I doubt it is an ingrowing hair, as there is a strong rooted hair directly sprouted from the source of the discomfort. Thank you.