This is totally normal, and it's hard not to get your feelings hurt by it as a Mother. I took maternity leave with my oldest son for 3 months, he went to daycare for a month and a half, he stayed with my Mom for two and a half months, and then was in daycare for 6 months. Now I stay home full-time with he and his baby sister (since my son was 13 months old).
When my Mom is around, he prefers her over me (even now that I stay home with him full-time and he doesn't see her every day ... she lives almost 2 hours away). They definitely have a bond, but he also knows that she's his Grandma and kids tend to like grandparents pretty well! ;)
It used to hurt my feelings when he would "choose" her over me, but I remember how close I was to my grandparents (who passed away when I was 17). I'm glad that I had that relationship with my grandparents, and am glad Luke loves his grandparents and gets to have a close relationship with them.
The only time I've said something (and we only had to talk about it once) was when my Mom was disagreeing with how we were handling certain situations with our son (he's two now, and can sometimes be a bear at this age). I just reminded her that we were her parents and that we loved him, but we would handle the discipline for our family and handle things the way we felt best. I asked her to please respect us and our parenting roles, and asked her to not ever do anything (especially in front of our son) that would demean our roles as his parents or undermine our authority. Things have been fine since. ;)
I know this is hard, but I promise your daughter knows you are her Mommy and no one could ever replace you! At such a young age, it's normal for her to cling to not only her daytime caregiver, but also her grandmother ... so you get a double whammy! ;) Best of luck to you!