Here's a good article:
https://www.backyardchickens.com/articles/mushy-chick-disease-yolk-sack-infection-omphalitis.64686/
The symptoms can vary, but their are examples include swelling, infected looking navel area, discharge, attraction to heat, needy cheeping, etc. It could still be...
I'm so sorry your chick seems unwell! You might get a faster/more responses on the injury/emergency section if you repost there.
How warm is your brooder currently? Is your chick trying to huddle with its hatchmates (cold perhaps) or is it all by itself somewhere in a corner (too hot)? Any...