Possible Miscarriage

Meron po ba dito na naka experience ng heavy bleeding with blood clots at 3 weeks, may nagpass na tissue, pero nagproceed ang pregnancy? I am currently at 3-4 weeks based sa hcg level ko, plus preg test shows positive result although manipis lng ang line. Sa ultrasound, wala daw evidence of sac sa uterus. I dont want to have false hope pero my OB still gave me heragest pampakapit just incase lumaban pa si baby. Baka daw kasi too early pa ung ultrasound kaya wala pang makita. Hope you can share your experience mga mommies ?