I am 39 and I have noticed a little bloodstaining on the inside of my bra cup. What does this mean?

Well, it probably means you're bleeding through one of the tiny ducts which lead down to the nipple. This symptom can be due to benign causes but it could also indicate cancer. Therefore, you must get your breast examined by your doctor this week. She will almost certainly arrange an urgent referral to your local Breast Unit. Good luck.