6 week check up confusion

I had my baby 6 weeks ago. Monday was my 6 week check up and my ob told me I'm not pregnant (test was negative) which was a breath of fresh air. She told me I could have my period any time. I have had spotting but literally no where near where it would be considered a period. And well I have been having sex since 2 1/2 weeks PP. I'm kind of freaking out I have had a few random gushes of white cm. I am wondering if someone has gotten told by your doctor that you are not pregnant at 6 week check up but you end up being pregnant before your first period?