Unusual hard mass in breast? 20 years old never get sore breasts, what could this be?

Carefully evaluate: Wait for your next period and check your breasts again 7-8 days after your period began. If it is still there, or especially if it got bigger see a specialist, start with your Gyn. If it got smaller, or if you can not find it, check it again at the same time next cycle and continue to follow this schedule in every cycle at the same time ONLY!