How long does HCG stay in your system PP?

I took a test 3 weeks 4 days PP and it was positive today I have had random spotting not a whole lot but red blood not brown and super mild cramps. I don't know what to think I will be extremely upset if I am pregnant again as it would be my 4th baby but it is my fault if so. I am just wondering if it could be left over hormones from birth?