Was the chocolate melted well? Or did you over heat the chocolate?
Sometimes, if you let the heavy cream boil too long, it will start to curdle and get like a film on the bottom of the pan. If that gets scraped off into your boiling cream (like while you're stirring), it'll get little chunks in your chocolate... could that be it?
This happened to me once, I have that the graininess has something to do with temperature changes either cold to hot or hot to cold that occurs too quickly. Supposedly you can fix it by re-warming the ganache and inserting an immersion blender into the chocolate and blending well. I have not tried it though.
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