What Causes Light Inter Menstrual Bleeding With Sore Nipples And Breasts?

Hi my name is Ashley. I'm 24 years old and have never had this happen to me. I was 6 days late last month, got it, then the beginning of this month had another and now today started. But the last two times were only three days and not so heavy, normally I bleed for 7 days three heavy two nothing and the last two days old blood. My nipples and breasts have been sore since before I was late last month and I have gained a little weight.