Please relax. Your child is two. Two-year-olds don’t yet fully understand relationships. Two-year-olds are still mastering language. It may be that your child thinks any adult woman is called “mother.” That doesn’t take anything away from your relationship with your own child. I assure you: Your child knows who his/her mother is.
A bigger problem is your distrust of your child’s father. To ensure that your child will have the very best chance to grow up emotionally healthy, the two of you need to co-parent well. You need to trust him enough to be relaxed when she is in his care. If you can’t to that, please consider getting into couples therapy with him. Relationships are built on trust. If you two don’t have that, your relationship will always be fragile. You deserve better. Your child deserves parents who love and trust each other.
I wish you well.