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.
It sounds like she just doesn’t want to be petted at the moment. I ‘ask’ my green cheek if she wants to be petted by waving my finger in front of her so she knows what my intentions are. If she appears fluffy, I will proceed to pet her. If her feathers are tight against her body, I will try again later. I would also recommend only touching the head and neck