I would wash her up (mild soapy water is fine, or a little Betadine or peroxide, well diluted) and see what is under there. Then an antibiotic ointment like Neosporin would be good if the chick has raw skin or a sore. Just be sure there is no "caine" drug in it, such as benzocaine, cetacaine, etc. Neosporin with pain reliever is fine.
I am wondering if this is an "infected belly button" (oomphalitis.) If not, perhaps there is something in the chick's environment that it is rubbing its belly on.