It sounds to me like you need to be clearer with her about your plans. I'm a working mom and I will ask for schedule changes based on different appointments, meetings, etc. But I ask _because_ I want to know if it's OK, and I will try to arrange something else if it's not. Recently a lot of changes have been going on and ( for example ) I asked my care-provider to stay a couple of extra hours and feed both kids dinner with very short notice! She cheerfully said she could, and as far as I know there were no problems. If it had been a huge problem with her and she was smiling to hide her grimace, I'd be horrified.
It sounds like your 'mom' is asking you for changes, you are saying "Yes", and then you are irritated at the trouble you have to go to to rearrange things. You probably should have a discussion about what your 'default' schedule is, and how you may not be able to easily accommodate changes. The flip-side to that is that she may decide she needs someone else with more flexibility.