What Causes Inter Menstrual Bleeding Along With Abdominal Cramps And Nausea?

I had unprotected sex a little over two weeks ago, I am on birth control and he did pull out. I started my period as expected about 2 days later. On Sunday, exactly 2 weeks after my period I started bleeding lightly and then started to bleed a little heavier as the days have gone by (it is now Tuesday). I am cramping and breaking out as if on my actual period and get a little nauseous at times..I m not sure why this is happening as this is typically the time in which I d be ovulating . I am currently under a lot of stress and did just switch the time in which I take my birth control so I m not sure if this has something to do with it and this is breakthrough bleeding and im just adjusting or if I m showing signs of pregnancy. I am quite concerned.