Oh please do forgive all my typos! I wish I could edit text. My keyboard is terrible!
Is the lump red, sore, inflamed? If so it is possible it's an abscess. They are very painful and need draining and antiseptic, and possibly a course of Amoxycillin.
But or course it could be something else.
Go get this lump checked. If it isn't an abscess, I would advise a needle biopsy. And if the results of that are 'inconclusive', then follow up with an ultrasound scan or whatever the vet suggests.
My own dog developed a sudden lump near her tail. It literally came up overnight. I thoght it couldn't possibly be cancer, as surealy cancer doesn't come up overnight!
Well it was. It was Hemangiosarcoma of the spleen, which had spread to form subcutaneous tumours.