It is impossible to provide a specific diagnosis over the Internet. However, in general, most early breast cancers do not cause pain (although a person can still have a breast cancer located in an area of the breast that also contains tender or painful fibrocystic breast tissue). Although this lump may be a benign cyst, there is no way to know for sure if it is malignant or benign without further work-up.

You did not state your age, but if you are under 35, your should ask your doctor to examine you, and also to consider ordering an ultrasound of the breast. If you are over 35, then your doctor should examine you and consider ordering a mammogram and, if the mammogram does not provide helpful information, then an ultrasound can also be performed as part of the initial work-up.

Hopefully, this is a benign lump, but it is not wise to ignore it if it is not clearly resolving. As for your family history, unfortunately, 70 to 75% of women who are diagnosed with breast cancer do not have a known history of breast cancer in their families.

Sincerely/ Robert A. Wascher, MD, FACS

http://doctorwascher.com

As there is just a lump that is painful may indicate that you have a benign (Inflammatory or fibroadenoma etc)lesion however it is better that you get examined by your family Doctor who can suggest some tests after conducting a physical examination.
There are range of lab tests besides a mammography with or without sonography that may be better for characterizing the lesion and rule out possibility of cancer if that is suspicious.
Breast tumors may be painful or painless.Fibroadenomas are related to period as you have mentioned and are simpler to treat too.
If necessary a Fine needle aspiration cytology also may be suggested by your Doctor to rule out possibility of Breast cancer.

Thanks so much for your response. And to Dr. Wascher, I am 21 years old. I will contact my doctor today and make an appointment. Thanks again, and wish me luck.

Thanks and I wish you good luck.
With best wishes from Dr M Chowhan