She is not spayed, but not a breeding dog just a pet. I have another pet, a neutered male Chihuahua and they do mate sometimes when she's on heat, but her heats have been sporadic and not regular as she gets older.
No the vet didn't do any tests.
The lump appeared quickly over the last 7 days I think, but could have started small and got bigger over a longer time and I didn't notice it.
Thank you very much for your advice. I will do as you suggest and hopefully get those tests done.