B.B.
I had the exact same thing with my 2nd pregnancy (and not the first). Exactly like you're describing....nowhere else except my belly button. My dr told me it was the way the baby was positioned and making everything push against my umbilical. She said she had the same thing in her pregnancy, so she knew what I was describing. She told me it was okay, and not to worry too much, but if the pain became worse, to call and let her know. I would still call your dr and maybe talk to a nurse, though, just to be sure that's what it is with you. They may want you to go in, they may not. I didn't make a special appt just for that, since I had one of my scheduled appts the week I started feeling them, so I just asked then.
I asked more questions in my 2nd pregnancy than my 1st! I had such an easy 1st pregnancy, and my 2nd wasn't difficult, but definitely different! I didn't feel as good the whole time, and had new and worse symptoms. So don't feel weird asking questions even though you're not a FTM;-)