Massive bleed - needing reassurance.

So I’m 29+4 today and on Saturday night I was sitting on the lounge and started bleeding out of no where really heavily, no cramping, no pain just bright red blood gushing out of me. I called an ambulance right away and the paramedic on scene estimated by the time I go to the hospital about 500ml of blood loss.

Once I was at the hospital it was immediate ultrasounds and tests, everything came back clear. No placenta tears, placenta well and truely clear of the cervix, cervix was long thick and closed. Baby was well, movement normal, heart rate normal all the good stuff.

I’ve been in hospital since then being monitored constantly, the bleeding has reduced significantly with only one small episode on the toilet this morning. Otherwise all bleeding is dark brown/old. No pain, no contractions, minimal cramping.

Has anyone else experienced this in their pregnancy/previous pregnancies?

This is my 5th pregnancy, 2nd child, my first 3 were first trimester miscarriages...

I’m just stumped and so are the doctors as to where this is coming from as the ultrasound was clear and there’s no growths or anything obvious around the cervix??