You cannot be pregnant if you have not been sexually active for 6 months, and have been having regular, normal monthly periods throughout this time.
You are still a very young adult, and your breasts will likely continue to enlarge and change over the next few years. Although I cannot offer any specific diagnoses over the Internet, young women frequently have tender and fibrocystic breasts, and often these symptoms are more pronounced around the time of a woman's menstrual period. (However, the presence of a lump in the breast should always be evaluated further by a physicican.)
Sincerely/ Robert A. Wascher, MD, FACS
http://doctorwascher.com