What Causes A Scanty Menstrual Period?

I had my last period cycle completed on 17th August and on 23, 25 and 30 we had intercourse unprotected and my husband came inside me. We both are medically fit. Now my period is about to start on 11th Sept. But on 13th Sept there was some spotting when I wiped it, it was during the day time and evening also there was a spotting. Today I went to office and used only one pad during the entire day and it seemed there was flow, in the evening also there is also bleeding but very scant. I am in a fix whether this is normal menstural cycle bleeding or implantation bleeding. Please help me out in this matter.