Implantation ? Pregnancy ? Period ?

I had unprotected sex the day after my period and had a spec of blood when I got in the shower two days later which came with mild cramping. Only that spec of blood, nothing more and nothing less. Last Tuesday I had unprotected sex on the last day of my fertile window. He pulled out but however since Sunday I have been experiencing VERY BAD cramps, almost like period cramps and sensitive boobs. Today, Thursday I had a small pink spot of blood in my underwear. And I'm still having these bad cramps that I had since Sunday. What could this be ?