Help! 2 weeks duckling suddenly cant walk and has a red growth out of bottom

[Yaras3]

One of my baby ducklings suddenly cant walk on his right foot I noticed yesterday then today I saw a red pimple-like thing coming out of bottom area - eating and drinking fine seems to clean up fine still loud but I am very worried. Also today seems that both legs are weak I am not sure is it because of sitting long or is it both legs affected but cant stand at all now

 

[Birdsarelife]

Can you show pictures?

 

[Yaras3]

Can you show pictures?

 

[Miss Lydia]

Is your duckling pooping? What are you feeding and are you using any form of niacin?