Hi there. Thank you for using Petco Pet Education Center, formerly Petcoach. Sorry Cookie is having this issue.
She really needs to be seen by your vet as soon as possible. That is a tumor but it is very difficult to tell exactly what type it is with just a photo. My best initial guess would be a benign histiocytoma but again, there is no way for me to know this for sure.
I suggest you get her to your vet for an exam. Hope this helps. Best wishes.