12 weeks passed large clot

I went to the bathroom this morning and I felt a gush of water and a huge clot came out. It was about the size of a golf ball and shaped funny at 1 end. I just knew I had lost my babygirl. I'm at the ER now and they did an ultrasound. The baby is still there, and her heart is beating strong at 167 bpm. She's moving around and everything. They said my cervix is closed. They can't explain what it was. The Dr said it could have been a subchorionic hematoma. Can any of you ladies that have been in a similar situation give me any insight on this? I'm so nervous and scared.