This is not to scare you because it really could just be the muscles thinning but my cousin actually had to have surgery on her belly button area after her third pregnancy and had to wear a wrap all of her fourth pregnancy. I would highly recommend starting to where a belly support wrap. She wishes she would have known what could have happened before it did. She still struggles with the area. I think it is called a belly button hernia but not certain I wish I could remember. I do remember having more pain with my second and worry about my third pregnancy now and how my muscles are going to feel in a few months. Hope you feel better soon and congrats on your third.