Scary bleeding at 21 weeks

Has this happened to anyone else?

What could it be?

It's not abruption or previa.

On the 26th I woke up to some heavy bleeding with clotting. I immediately woke my husband and went to the ER and was sent to L&D triage. They ran labs, did a cervical check, pelvic exam, and transvaginal ultrasound. They also checked my vitals and baby's vitals and everything came back normal. Cervix was/is closed. Cervix is a good/desired length. I was around 21 weeks when this happened. I was sent home and told to take it easy and no intercourse. The bleeding slowed down after that until the 30th when a clot the size of my pinky came out. I'd never seen a clot that size before so I went back to triage. The ran all the same tests. Only thing different was they did internal AND external ultrasound to get more angles and look at more things. There is no evidence of abruption or previa. They couldn't tell me why I bled. I am now almost 23 weeks and my little boy is still strong and moving around like normal. But this is my 7th pregnancy and it's the first to make it past 8 weeks. He's my super rainbow baby. I'm so scared.