Usually hormonal changes are what causes sore breasts. I get some soreness before AF too, but during pregnancy it's 10x worse... I could barely stand a shirt touching me.
i used to get really painful breasts before my period. you could be getting your period soon, are you on any birth control pills? or birth control methods? that could do it too. also, your breasts get VERY painful beyond belief if you are pregnant.
Could you be pregnant? Or is it around that time of the month??