Subchoronic Hematoma

I had to go to the ER last night cause I was in pain, they did an ultrasound as checked the baby. The dr said the baby is going great and the heart rate is 161. When he discharged me he said I have a subchoronic Hematoma and I need to prepare myself of a miscarriage. I don’t understand why the baby is perfectly okay and the Hematoma is small. I made an appt with my new ob tomorrow to go over everything. I was just wondering if anyone else has gone through this because I’m a complete wreck.