What Causes Back Pain, Breast Heaviness With A Delayed Period?

good day doctor I am 20.I did several pregnancy tests that happened to be negative.I even went to see a dcotor but he told me there was nothing.But I still have that pain down my back and my breast is heavy as if i was pregnant and also I was supposed to have my period the day before yesterday.now i'm lost and i'm scared help me please!