Thumper had her needle biospy done with novicane (like dentist uses for numbing teeth) and she didnt feel anything from the experience.

Please learn from my present experience with Thumper, that "let's wait and see" is not what a vet should be saying when there is a "lump". You cant tell anything from justing looking using the naked eye, I dont care how qualified a vet, they arent super natural people who can see through objects.

I'd get a needle biopsy.