R.J.
If your child is the only child she cares for, you can tell her you want to socialize her and get her into a place with more children. If not, just tell her that you are concerned about the way she is behaving when you drop her off, and that its not getting any better and its hard on you to leave her like that. Maybe your child is just bored, and you want to try a new environment with new faces and a more structured environment etc. If you are sure that the sitter is not causing the child to scream, assure her that you are sure its nothing she has done, but that your child is just growing up and changing and you want to do what you think will help her grow the most.