so you think this may be breast cancer?how far along is she? It sounds to me like her breasts are producing milk and one breast may be swelling or i pain due to the milk getting blocked or backed up.it also could mean that her breasts have 2 diffeent levels of hormones in them making one larger than the other but at the end of her pregnancy they should level out!

also I found this:

And I quote "If your wife continues to experience pain, has a high temperature, flu-like symptoms or any reddened areas on her breasts, she should consult her GP or midwife for advice".