I know it is only a week old, but how swollen is it? The barbell going inside the hole could be either an illusion caused by severe swelling, or it could be rejecting.
I agree on the fact that you need to go back to the piercer. If it is just caused by swelling, you need to have him change your jewelry to a ring. It should have been pierced with a ring in the first place to avoid this type of thing. If it is rejection, you may just have to take it out and let it heal.
Along with wearing looser jeans as was said before, you also need to wear long shirts to avoid your shirt bottoms from getting caught on the barbell.
Belly button piercings are one of the most prone piercings to irritation and infection. Some of the things that you wouldnt think would effect it actually will. Be very careful with it, and remember, only touch it when you clean it. Dont sit around and play with it because that will just make matters worse. When you clean it, make sure your hands are clean. Dirty hands will also make matters worse. Good luck and let us know how it turns out!
