Sudden Heavy Bleeding

Absolutely terrifying night. I am 12.6 weeks and had to pee; when I wiped, my tissue was covered in bright red blood and jelly like textures. I looked in the toilet and it was bright red with blood in addition to a large clot I passed. Raced to the ER bawling that I may be miscarrying. Four hours later, my baby is moving, healthy heartbeat, no source of bleeding could be found even though my urine sample was bloody, and placenta previa was detected. Any ladies have anything similar? Any ideas as to a potential cause? I am also Rh negative and it was crazy I had to ask for the rhogam shot on my own. I miscarried 3 years ago and was not given the shot. I'm also stressing I may be Rh sensitized.