There was a small drop of blood that I believe came from my left nipple. The breast ultrasound was negative so I never bothered coming back for the result of the mammogram I took last November 2015. Is the result only relevant within a time frame from the date of the procedure?

If the discharge is bloodstained and from a single breast, the cause is likely to be a duct papilloma. A papilloma is a harmless wart-like growth, usually about 1-2cm in size, found inside one of the breast ducts. The papilloma is usually just behind the nipple and can cause fluid or blood to seep out of the nipped. It's usually non cancerous but to have peace in mind, no harm to go to doctor to check...