Why is One Breast Hard and the Other Soft?

As you can see from the answers, the Real Self team of excellent plastic surgeons feels  you have given insufficient information to understand exactly what your question is.  Therefore, I am going to make certain assumptions in order to answer the question I believe you are asking.  I am going to assume that you had a breast augmentation, and that after the breast augmentation, one breast has gotten much firmer than the other side, and as it has gotten firmer, it has also gotten painful.  Assuming that this was not immediately after surgery and there is no bruising, I am going to assume it is not a hematoma, and that it is, in fact, what is called a capsular contracture on the hard side. 

The body's response to a foreign body, be it a splinter in your finger or a breast implant on your chest, is to form a scar around it.  This scar is your immune system's way of protecting you from this foreign object which is seen as an invader.  When this scar or capsule starts contracting inward, it forces your breast implant into a smaller and smaller space, which moves upward and becomes rounder and harder.  The nerve endings that surround the breast can sometimes be trapped within this scar capsule, which is normally the cause of the pain with the hard breast. 

Why this capsular contracture occurred just on one side and not both is the $64,000 question.  I believe the reason the capsular contracture happens on one side and not both has to do with deferential inter-operative factors working on the two breasts.  Blood around an implant and non-pathogenic bacteria around the implant or in the implant pocket are the two most common inter-operative causes of a capsular contracture. 

You need to return to see your plastic surgeon as he (there are no female plastic surgeons in Hawaii) has several modalities to decrease your pain and, potentially, cure your capsular contracture.

Why is One Breast Hard and the Other Soft? This may be an indication that you have formed a capsular contracture on the harder side.

Sounds like your right breast has developed a capsular contracture (internal scarring).  You should check with your surgeon to determine the manner in which to proceed.  It is correctable.

Dr. ES