I always recommend having a necropsy done on stillborn foals at a state lab or University--they can test for viruses like Rhino, determine if there are defects, and do cultures. I know this isn't available for everyone, and sometimes the answer is no answer.
Was the filly full term? How did the placenta look? There are soooo many causes for late term abortions and stillborns.
I can tell this is still weighing heavily on your mind more than a month after the fact--I sure wish I could give you some answers. All I can offer is a hug and a sounding board if you need it.