Dogs paw is red and she is limping?

This has been going on for a week now, I just looked at her right back paw and it is very red the part where its sup-post to be black is red and its even red up her leg a little bit. it is making her limping and hard for her to walk. What is wrong with her. and is this something that heals on its own or do i need to take her to a vet?