progesterone starts to act around the time of ovulation

Progesterone causes water to be retained in the body. The main reason for this is to prepare the uterus for a possible pregnancy by making the uterus rich in water and food for the anticipated pregnancy. Unfortunately, this same effect is felt throughout the body. Storing of water in the breast tissue causes it to become stretched leading to painful breasts and sore nipples.
I stole this of the intermernet. Seeing as TF and ovulation are around the same time, then it could be a bad case of water on the boob.