First ultrasound. Spotting

So I had my first official ultrasound today at 6 weeks and 6 days after my scare at 4 weeks. There was a strong heartbeat detected today of 129bpm and that really reassured me. I had an abdominal ultrasound. Well, last night I noticed I felt really tender down there and was very slighly spotting. It seemed to have let up. But today after I got home I noticed more spotting. I'm slightly cramping and trying to remain positive. I read online that after a hearbeat is detected, the chances of losing baby are low. Everything looked great on the ultrasound and baby and all measurements were good. Has anyone had this before? I need reassurance it'll be alright. I had one miscarriage before this baby and can't deal with losing another...