For all the education and awareness on breast cancer out there, sometimes I think the docs should be the ones being educated. 20 is NOT too young. Not common,perhaps, but not too young. I also agree with Mamakaycee's suggestion...see your sister's doc and get his opinion. Until you are sure in YOUR mind, keep asking questions and make them convince you.
Crisci - I am much older than you with a history of breast cancer in my family so my situation is different than yours, however I went through a similar journey with people telling me that the lump I clearly felt was either not a lump at all or not anything to worry about. When I finally got someone to agree that I had a lump that had to be dealt with, I was faced with the option of either biopsy or removal. My surgeon suggested removal due to my history of cancer. I agreed, and as it turned out I made the right decision. What I'm learning is that you have to educate yourself, trust your own instincts and be assertive about your care. I think Mamakaycee's suggestion on getting your sister's Dr.'s opinion is a good one.
Don't fool around with this lump if it is getting bigger. You are not too young to have cancer. If you can get to a major medical hospital with a breast center, do so.
I also feel that for any woman, it is never too young. My cousin had Breast Cancer @ 20. And you could never be sure if it is benign or malignant unless a biopsy is done. A sterotactic biopsy done by the correct surgeon is a great way to go. Done right in most surgeon's surgical room and only leaves a dimple. Women know their bodies better than anyone, so if you feel the lump, take the mammo to a surgeon and have him/her look at the mammo and decide for you. I had breast cancer and after a few years there was what the radiologist, then the surgeon, said was a "solid". I said just cut it out, and it ended up just being a scar tissue, but I wanted to be sure. Why don't you take the mammo to the surgeon your sister went to? Good Luck!
You are not too young. I was 36 and pregnant when I was misdiagnosed with a lactating adenoma. I had stage 3 breast cancer when it was finally removed 5 months later. Some cancers grow really fast, others really slow.
Do the biopsy to be sure. It's really the only way to know for sure.
Be your own advocate, find a doc who is listening. Even if you turn out to be wrong, at least you will know and it will be documented on what it was.