Please don't rub your bird's back or belly. These are intensely sexual for a parrot, and, if accepted, is likely to make her think that you are her mate. This can lead to aggression directed towards others, hormonal behaviour and egg-laying. These can be major problems which you want to avoid, and definitely not encourage.
Your bird is trying to tell you to back off. This is good! She's telling you that she only wants to be touched on the head. This is a nice, friendly place to stroke or gently scratch - parrots who are friends but not mates will preen each other there.