What Causes Swollen And Painful Breast When On Lutenyl?

It is a second months that I am taking Lutenyl prescribed for 15 days starting from 11th day of my cycle to try to make thinner my uterine endometrium. As it is 17mm thick and I have been trying to concieve for 10 months without success. Done Endometrium biopsy which came back fine. How lond do I have to take Lutenyl to have an effect? Or there is any other medicine more suitable? Also I while I am on Lutenyl I have swollen and very painfull breasts. Is it normal?