edges look muddy after editing

On just some of the photos I edit, the edges look muddy.  I've attached the original photo along with the edited photo to show the difference.

What am I doing wrong?  This doesn't happen  with all images but when it does, it's ugly.

I remove the background by first, using the object selection tool.  Then I create a mask of the selection.  From there I use one of my brushes that help me refine pet hair.

Thanks in advance for all help