Hormones can be different each month your period comes. Weather, water retention, what you eat can all affect what your body does to prepare for the mentrual cycle. Then when you have a baby, your body does even more interesting things :)

I have had this happen before, but usually it's both of them and they seem to get bigger and sore about a week before my period.  I've been taking a herbal supplement to help balance my hormones and that really helps alleviate the soreness