Brown spotting 7 weeks

I've had this spotting on and off since

before I found out I was pregnant. l've had

an ultrasound and my baby is measuring

perfectly with bmp of 115. They did find

second sac but it was empty. Ever since my

ultrasound, I've been having more spotting

and it's getting darker. Has anyone else had this happen and had it be completely normal? My OB keeps saying that as long as I'm not bleeding I'm fine but I'm just worried. My last pregnancy was so hard, im having serious anxiety this time around.