Heavy Bleeding at 21 Weeks

I really am searching to see if anyone has been through something similar...and then what was your experience/result.
Last Thursday I started to bleed bright red blood heavily. I went to the hospital and the doctor was able to stop it and noted that I was dilated to a 1. We were kept overnight for observation and I have been on bedrest since. Ultrasound didn't reveal any reason behind the bleeding.
Since this happened I do still spot red blood randomly, but it has not been heavy. I was referred to a Perinatologist and saw her yesterday. On her ultrasound she found a large clot in my cervix but didn't find any indication of where the bleed could have come from. My placenta looks perfect, baby is perfect, everything else looks completely normal. 
I see my regular OB on Tuesday to find out the long term plan and then see the perinatologist again in 4 weeks. I just wish I had answers as to why this happened...I'm so scared this will keep happening! 
Does anyone have experience with this? I'm now currently 22 Weeks. For the record I'm posting anonymously due to my job.