what should I do?

Hi - This soreness could be from any number of causes; usually, soreness in one or both breasts is due to a hormonal issue. Or to fibrocystic change, which many women experience. There's a very rare form of breast cancer, inflammatory breast cancer, that can start as soreness in one breast - it usually progresses, fairly quickly, to what looks like an infection: your breast is swollen, red, hot, painful and sometimes itchy. 

For now, keep your eye on it; if the soreness doesn't abate within a few weeks, then best to see a doctor about it. If it gets worse quickly, and seems infected - see a doctor to rule out IBC. As I said, though, this is probably hormonal. Good luck - PJH