Sometimes doctors know right away what's wrong and can prescribe a treatment that will take care of the problem.  Often, however, they diagnose by a process of trial and error.  Your doctor thought the most likely cause of the itching was a yeast infection, which is a common cause of breast itching.  Now the ball is in your court to let him know that the cream didn't help, so that he can move on to a Plan B.  Be sure to let him know about the pattern of more intense itching before your period.  That might be an important piece of information to help him figure out what is going on.  I hope he can help you find a solution.  In the meantime, try not to scratch.  I've found that steady pressure on the itching spot sometimes helps relieve the sensation without the scratching that can make the situation worse.