Possible Miscarriage

so on Christmas eve I found out I was pregnant with baby #2. everything has been going fine up until yesterday when I started having severe cramps in my pelvic area but no bleeding so I wasn't to concerned until while I was making love to my other half we noticed blood and so I went to the er. as of today I'm suppose to be 6 wks 1 day and the ultrasound was no showing where I am suppose to be for this many weeks it's showing earlier and my hcg levels are pretty low. my last period was Nov 21st and I'm pretty accurate with my cycles. so the bleeding has completely stopped and I was told that I could be having a threatened abortion and to follow up with an obgyn. the bleeding was not heavy it was moderate. I had bleeding with my first pregnancy which was a blood sac in my uterus but my son was fine and was measuring where he was suppose to be then until 22 weeks he stopped growing and I delivered him via csection 3 days later at a day shy of 25 weeks. I'm trying not to stress because this is out of my hands. just was very excited to be pregnant again and I've had a miscarriage before this pregnancy. so if it happens again this time I'm going to see a specialist and find out why I can get pregnant but,my babies stop growing.