Breast Lump: What Could It Be? Ultrasound Negative - Seeking Information

Question
Im 21 years old and have a large, hard, solid, painless lump in my upper left breast. it seemed to have popped up out of nowhere!! no other symptoms though. i had an ultrasound today and nothing came up. my doctor, myself and ultrasound technition can all feel the bump- but it didnt show up on the ultrasound!!!! do you have any idea what it could be? it was suggested that i get a mammogram, but arent i too young? and if it's nothing why would i need a mammogram? would it be better to get a needle biopsie? i had a baby 2 years ago, could it be a clogged duct? i never breast fed him either. there is no spokenof breast cancer in my family (but my fam tends to be hush hush about such things). whats the difference btwn a mammo and u/s. does the refferal for the mammogram mean it's sever? ahhh. im worried!!!!!

Answer
At your age the risk of a breast cancer is very small. I think you should have a mammography though since something can be felt there AND ALSO a needle biopsy! Should be quite easy to do since it can be felt. With that we will know what it is. Also a breast MRI scan can be done. Mammography uses x-rays while the ultrasound works like radar but uses sound waves and their echoes to do it.