Need some positive thoughts...

I'm 8w 3D pregnant with baby #2 and have some dark brown spotting. I've had an ultrasound (at barely 7 weeks), but it was a little too early to hear/see the heartbeat. I never spotted during my first pregnancy. I haven't had sex or an exam in the last 72 hours. I am still experiencing pregnancy symptoms (light nausea, sore breasts).

I have an appointment on Tuesday for a follow up US, but I'm scared. Has anyone else experienced this?

UPDATE:

Thank you everyone for your kind thoughts. I'm fairly certain I've miscarried. Spotting turned to full on bleeding and clots. Trying to get in to see my dr tomorrow. I'm sad, but I know that this is so so common and it's happening for a medical reason. Thanks again.