Why is One Breast Hard and the Other Soft?

Without more information on what type of surgery you had, it is hard to know what is going on.  However, assuming that you had breast augmentation in the past, you probably have some sort of capsular contracture around the firm breast.  I would address this with your surgeon if indeed you have had previous breast augmentation.

Although your
history is somewhat limited, it does suggest the presence of a capsular
contracture.For this reason, it’s
important to consult your plastic surgeon and have a treatment plan formulated
to deal with this problem.Whenever a foreign material is
placed inside your body, it will generate a reaction from the surrounding
tissue.This typically results in the
formation of a fibrous capsule that surrounds the breast implant.With time, the capsule tightens and undergoes
contraction.When this occurs following
breast augmentation, we call it a capsular contracture.All patients have some capsule
formation.Indeed, absence of capsule
formation might result in the implant shifting.When capsule formation results in a contracture, patients may experience
hardening of the breast, distortion of the breast, and breast pain.Capsule formation can occur at any time and,
as time goes on, the incidence slowly increases.When it occurs, patients note firmness of the
breast; as it goes on, this may be accompanied by pain and progressive
distortion of the breast.The breast may
shift in an upward direction and develop an abnormal shape.The cause of capsular contracture is
not totally understood, although multiple theories exist.When capsular contractures occur, there are
several treatment options.In mild
cases, medical management and massage may be necessary.In severe cases of capsular contracture,
surgery may be necessary.This may
require release of the contracture by performing a capsulotomy or removal of
the capsule by performing a capsulectomy.Treatment has high satisfaction rates and most patients are ultimately
happy.

This depends on how long after your surgery it's been. If you are just weeks after surgery, then this could be due to swelling affecting one breast more than the other, which will gradually resolve in the next few weeks and months. If it's been several months or years after surgery, then this could be an indication of capsular contracture. Capsular contracture causes the breast to become firmer, tighter and elevated. You may also feel pain. Not having more details about your situation, the best advice for you is to see your surgeon for an assessment.