What Causes Soreness Along With Tenderness In Breasts?

I m 37 years old and have not had a period in 2 yrs due to surgery. 4 days ago my left breast became tender and increased to the point it hurt to touch or wear a bra. I have discovered my breast was full of milk or what seems like milk. I have 2 children who 11 and 12 and haven t breast fed in 10 yrs. What could be the cause of this?