I would say its the time spent with Mom. I think its normal I've had kids do this at different points during thier time with me. I have one little girl and any change in routine will set her off. She attends preschool Mom dropped her off instead of me she cried and whimpered most of the morning she doesn't even cry when I take her to preschool. Last year they were in swimming lessons one afternoon a week Mom got off early to come swim with she wouldn't let go of Mom the whole lesson whereas with me she was off to lesson no problem. Children go through changes and sometimes have bad days and even bad weeks, I wouldn't worry about it. Let Mom know it is normal.