I too (and so do all the women on my mom's side of the family) have fibrocystic breasts, so I feel your pain in never knowing what's what. As for risk factor--keep in mind that I am not a doctor or genetics counselor--but a lot depends on the age of your grandmother when she was diagnosed and if there have been any other cancers (specifically ovarian and breast) in the family? I recently went to a genetics counselor to see about getting BRCA tested and was told that based off of family history (all on my mom's side--grandmother diagnosed w/lymphoma at 45'ish, passed at 50; grandfather diagnosed w/colon cancer at 67'ish, passed at 71; and aunt diagnosed w/breast cancer at 35, beat it) my risk of getting cancer is the same as the general population. So hopefully that helps put your mind at ease.
Now, having said that I think that if you are concerned about the lump in your breast you should push for an u/s and/or biopsy. You are right in that it will be tough to get a mammogram at your age because the tissue is so dense it's hard to read. I finally (at the age of 29 3/4) had my first mammogram, which was a digital mammo, after much head banging. While I know it's not as good as u/s, it's at least useful to a degree, plus they don't do screening breast u/s's where I am. Since you have a lump you should have no problems getting in to have it looked at.
Good luck and let us know what happens!