Pick the Same Time
Because of the normal hormonal fluctuations in a woman’s body that affect breast tissue, it is important to select the same time every month so you will be able to distinguish between a normal change and something that feels different.
Do your exam toward the end of your menstrual period. The end of the menstrual cycle is the time when hormonal changes have the least influence on breast tissue, and the breasts are the least tender.
Choose a day of the month (e.g., the 1st or 15th of the month) and consistently perform your breast self-exam on that same day every month.
Source: John Hopkins Medicine