hi,thanks for query.Please do not worry,the breast pain
could be due to some muscular problem or hormonal
changes.Please try to recollect about whether you had something heavy to life on left side,some minor trauma or like that can give rise to this kind of pain.As you are saying you are having irregular periods
so this hormonal changes could again be the reason.Try to feel the area of pain with palm of your hand in standing position and then with fingers in sleeping position.If you can feel any lump or some hardness,then you have a little choice but to visit a doctor for treatment.Also look for some redness
over the skin.If not ,wait for some time ,apply some analgesic
over the painful spot and it should go.
wishing you good health.