Younger women tend to have much denser breast tissue than older women. This can make a breast-cancer very difficult to detect in a younger woman using standard mammography. You may wish to go outside of the standard mammogram and pursue 3-D mammography as well. The most difficult breast to evaluate would be one in a lactating female or a woman who was pregnant. You may want to start having mammograms potentially at 30 unless you feel a lump developing in your breasts. You also may want to consider MRI examination of the breasts. This however is extremely sensitive at finding “things“, however, the sensitivity can sometimes lead to unnecessary biopsies. Finally, since both your grandmother and your mother had breast cancer at such a young age, I would strongly urge that you undergo genetic testing. This would give you better information as to your risk. If you have the BRCA1 or the BRCA2 gene, you are at higher risk. I feel this would be especially helpful in your instance.
and breast cancer are not my areas of expertise. However, I'm fairly
confident that, given your family history, your are almost certainly at
higher risk. With that being said, I believe that you'd be a candidate for
screening MRI. I'd recommend for you to contact your local / regional
breast cancer center and have a consultation.
Wish I were more help.
All the best.