Tender Breasts. Taking Plan B can affect your next menstrual period. The hormones in emergency contraceptive pills prevent ovulation and alter the uterine lining.Because of this, you may notice some changes in the symptoms of your period. Tender breasts may or may not be a routine side effect of your impending period.After taking Plan B, the likelihood that your breasts will become sore or tender is high. The discomfort should be alleviated at the beginning of your period.ender breasts may or may not be a routine side effect of your impending period. After taking Plan B, the likelihood that your breasts will become sore or tender is high. The discomfort should be alleviated at the beginning of your period.