Was the surgery recently?
It takes a lot to botch a spay surgery unless a) the vet was inexperienced, b) the vet had been drinking or c) the vet is a complete idiot who doesn't check for cleanliness.
The vast majority of the time when a lump appears beneath the incision it's due to a seroma. A seroma is a pocket of serum/blood/body juices
that forms because the owner didn't follow the orders of keeping the dog inactive for the stated time frame. I've never seen a problem arise from a seroma other than a stitch or 2 pop out & us have to put a staple or 2 in. And our clinic does around 20-30 spays & neuters a day.
However, I wish you & your dog the best & hope the dog gets to feeling better soon.
EDIT: I just reread your post & the part about the vet saying it was a hernia due to dirty materials doesn't make a lick of sense to me. Unclean/unsterilized tools can most definitely cause infection. But a hernia results when a there is a hole in the abdominal wall that allows the "insides to become outsides."