I have never heard of it being a sign for miscarriage before. A lot of women get constipated during pg, but just as many women get diarrhea throughout their pg. I was one of the diarrhea women
I had it almost all throughout the pg. My OB said it was almost as common as constipation. So I wouldn't worry about it much. In fact, worrying a lot of times makes diarrhea worse!
2 BIG signs of miscarriage are cramping and bleeding together. Other than that, there aren't really any other big signs that the doctors tell you to look for.
Good luck and try not to worry!