Does she have a lump at all?
I'm wondering if she has a hernia.

She says no.

You might want to consider taking her to the doctor to get it checked out.

I would also consider that she might have a parasite which is very common for kids. One of the most common symptoms is pain in the belly button. She could also be itching her bum alot or grinding her teeth. There are kits you can buy to help cleanse the parasites out of her system and that should make a huge difference for her.
Good luck!

does she still have her appendix