Since you have got clinical and sonographic examination done nothing much to worry about cancer...and its very rare at your age...Nipple pain may also start outside the breast... in the chest wall, muscles, joints or heart, for example Cyclic breast pain appears to have a strong link to hormones and your menstrual cycle. Cyclic breast pain often decreases or disappears with pregnancy or menopause...consult nearest surgeon they will help you if its not cyclical one..