My dog has a small ball on her back

I found it recently and was watching to see if it was a mosquitoe bite and would go away. It hasn't and was wondering if anyone had an idea of what it may be? Do bites normally take a while to heal or could it be a tumour?

I think you should have it checked out by a vet. If it's small and not growing , I wouldn't panic , but it's always better to have it looked at.