What Causes Swollen Veins In The Breast?

I turned 40 in February. I've had a lumpy and painful breast off and on for several months. I had my first mammogram 2 months ago, and they also did an ultrasound and found nothing. Now, the left side of my breast is very sore, and there are swollen veins. Now I'm wondering if the painful lumps under my nipple are also swollen veins. I produced ALOT of milk when I was nursing my boys, and this was the breast that always gave me trouble - mastitis, engorgement, etc. Can you develop varicose veins in your breasts? Any thoughts? Thanks