Not calling mommy

My 3yo uttered mommy when he saw my mil last night after three weeks not seeing her. He never call me mommy before. I thought maybe it's just a fluke so I tested him again with mil photos and he keep saying mil is mommy. I'm feeling really sad and disappointed as I'm the one that take care of my child. Mil likes him to call her in Cantonese "mama" which sounds very much like mama but in different tone. I teach him grandma as in tv cartoon but he forget everything. Should I be concerned?