I'm sorry you're going through this, how stressful. It's really hard to enjoy a pregnancy when this happens. The bleeding and clots could be from a subchorionic hemmorhage, which doctors can't always explain the reasons for (technically, it's blood that gathers between the placenta and uterus). The baby's heartbeat on ultrasound is a great sign. This could also be a miscarriage, which I truly hope is not the case, but miscarriages, like pregnancies, can be different, so the ones you had with no symptoms might not be the way this is, if it is in fact a miscarriage. Sometimes women bleed during pregnancy, clots and all, and things are perfectly fine, so hold on till you get a call back and get checked out. I have a friend who had similar bleeding after another pregnancy resulted in miscarriage, and she was very scared, but ended up with a healthy baby who is now 5! Hang in there... hugs!
ETA - So sorry for your loss. I know how thankful you are for your kids but they don't erase the loss of this one... I have been there. Take the time you need to grieve.