HI, thanks for using healthcare magic Mirena is a hormonal form of intrauterine contraception that does decrease menstruation. There are persons, who though they do not get a period with this medication , still experience the other symptoms of menstruation such as breast tenderness. Though the product does have a suggested duration, unfortunately this is not exact and it can vary slightly from one person to the next. Soreness of the breast is a common symptom seen prior to and during a period. It is likely due to either the hormone in the mirena itself or limited period symptoms or wearing off of the contraceptive effect though this would not be immediate. if you remain concerned, you would need to see your doctor for an assessment I hope this helps