Chick with small bump on bottom

[Amsignorelli]

I have a black silkie (I believe seven days old) who seems perfectly healthy and acts just like the other 3 I have. Eats and drinks, may be the smallest chick though seems to have a small bump on her bottom ( I thou was pasty butt) so I clean. It is not pasty butt but a small bump on her bottom. Should I do any thing or just leave it alone? What could it be? Thank you in advance

 

[Yep I Talk To Chickens]

Pics please

 

[Amsignorelli]

Pics please

I just cleaned her up. It keeps catching her poop

 

[Amsignorelli]

Yesterday there was no different color it was all black