What Causes Irregular Vaginal Bleeding While HPT Is Negative?

Hi I had unprotected sex on march 9 and I took norlevo. My period was on March 15, two days after the date I expected. They last for 6 days as usual. On March 23 after intercourse I had some bleeding but then it stops. Today during sex I start bleeding again. It looks like I m having my periods. I have had regular periods always. What should I do? I took a pregnancy test and it become negativ.