I don't think I've ever called my grandparents grandma and grandpa. I only use grandma and grandpa when when I'm referring to them in the 3rd person.

When I was a kid I had a hard time saying grandpa and grandma. Ever since I can remember I've called my grandpa "PAPA". Grandma's simply "Tutu" or "Lila". Tutu is Hawaiian and Lila's Filipino. When I was little, instead of saying "Lila" (because I couldn't say it), grandmas were "YAYA ISABEL" and "YAYA MERCEDES".

I don't know why, but I still call my mom "Mama" or "Mommy". Dad's still "Daddy". I don't see that changing anytime soon.