She's my only dog I've every had, and she's been a huge part of my life. I am so worried we're taking her to the vets tomorrow, but I can't even think about if it is somthing serious. She's eight years old and arthritic so that only makes me more scared....

 

This is her.

Hopefully it will be fine. it is really common for mature dogs to get lumps, usually they're just fatty deposits, but the vet will be able to tell with a small biopsy. Our dog, Holly, is 13 and and has lots of lumps. they don't bother her at all.