T.,
I go round and round with the same thing with my mother. She watches my son for me while I work. I also have a 3 yr old niece who she use to watch during the day. Her mother recently put her in daycare because of the same problems. She's a wonderful grandmother and I appreciate her help in every way, but sometimes she just over steps the line. She, as well as your mother, makes it seem like she is doing me a huge favor by watching my son - but when I found a sitter temporialy, she freaked out and said that she missed him way to much. It's one of those touch and go things.
Have you tried explaining your feelings to her. She may not realize that she is making you feel that way. I've just sat down and spoken with my mother several times on the issue, and she is slowly getting better. Maybe she's feeling that you don't appreciate what she is helping you with (we all know chilcare is horribly expensive). She may just need that little bit of reassurance. I know that is all that it took with my mother.
Best of luck!