I got my double bellybutton (when you have the top done double bellybutton would be the bottom) pierced a couple weeks ago. It was fine for about 2 weeks and then it really started hurting. I lived with it hurting for another week or so and I just took it out but it still hurts. Was my body rejecting my piercing?
It sounds like that may be the case. It kind of sounds like what happened to me. I had mine pierced, and the top hole was still red after about a month of having it and it was a little sore. I thought maybe it was infected but then I went back to the place where I got it pierced and they said it was migrating. Migrating is when the body doesn't like where the jewelry is and it moves it to where it feels better. It took almost 8 months to stop hurting and not be red and to completely heal, but it was worth waiting that long. Now my piercing is really where it used to be because it migrated, but it still looks nice.
Now for my nose piercing story. I ended up taking my nose piercing out 5 months after having it done because I didn't like it, and it hurt for 3 days after taking it out, so that may be why your belly button piercing hurts.
So all in all, either your body was rejecting it, migrated it, or it was just healing and your body reacted worse than most.
Good luck and I hope you can find out what went wrong!