HELP!! My frozen buttercream has these crazy clumps of what tastes and feels like clumps of granulated sugar. I make plain old crisco and butter with powdered sugar buttercream. Any insights???

Did you sift the icing sugar before adding it to your Crisco? If not that may be the problem. Sometimes the sugar has hard lumps in it and they don't break down in the mixer. That maybe what your seeing now that it is thawing. I freeze buttercream all the time with no problem. Always sift, those little buggers are hellish if you need to pipe with the icing, they get stuck in the icing tips. Well that is my best guess. Sift first, hopefully next time it will be fine. Hope that helps. Good Luck and Happy Baking.