Why does my breast hurt after my period?

One of my breasts started to hurt a week before my period when i touched it. and it has now been 4days after i finished my period and it still hurts i get random pains in it when im sat down and it varies one day it will be not as bad and then another day it will hurt and it sometimes feels like the pain is going to my back it is really worrying me is it just my body changing or the breast actually growing