6 Weeks Belly Button Red

I've noticed the past couple of days that my 6 week old's belly button looks a little red. It's not bleeding or crusty at all, but my husband says he thought it was stinky. I noticed that it started after I took her onesie off at night, and she sleeps in just her diaper in our bed because she gets too warm otherwise . I wonder if her diaper is rubbing on it, or if the oils from my body are getting in it, causing an infection or something? It's just the inside of her belly button, it looks fine when it's not sticking out. Anyone else experienced this?