PERIOD OR PREGNANT

 started my last period on the 8 and finished on the 12. I had unprotected sex on the 10. My cycle is average 31 days. It is now four days to my period and I have this (day 28). I had terrible cramps so I took a ponstan. 

Is this implantation bleeding or menstrual because this is a fair bit lighter that what my period would usually be.