Visited doctors about a sore breast, was told there was a hard mass of tissue but said it wasn't worrying her, what would cause worry?

Wrong diagnosis!: Let's just say for sake of argument that she is wrong. Who has the problem? Unless your doctor can prove that the mass is not a problem to your satisfaction, it is your life that is being "bet" on the correctness of her "no worry" reassurance. I'd ask for a second opinion with another doctor; maybe a biopsy to make sure the mass is benign! 20yo can and do get cancer (rarely, but possible)!