Missed miscarriage

Kylee
Went to er last night from pelvic cramps and a little blood. Passed a large clot at home..... Ended up with a missed miscarriage :( baby stopped growing at 8w6d and I should have been 11w today :(
 I don't understand! We went at 8w3d and heard a strong heartbeat. What can change in a matter of days? We are doing the genetic testing and I hope we are able to have some answers! 
First pregnancy, we will try again but how am I not supposed to be scared!