What Causes Acute Stomach Pain, Heavy Bleeding, Clotting With Tissue Type Discharges?

some years ago i had two weeks of brown spotting/discharge around the time i was due on my period i had unprotected a few times b4 that. then about a week two weeks later i hadthe most horrific pains ever like a period but more intense an heavy bleeding. the pain was in my stomach piercing through to my back an within two to three hours i started passing red big blood clots an then loads of grey stringy tissue was coming out too with the clots. how likely was it that it was a miscarriage? i never went the hospital as i was really scared an just went through with it at home . im now 20 an thats the only time thats happened to me with the blood clots and tissue an ive had my periods for over 8 years