What is the lump on my dog's gums after tooth extraction?

Hello, I am rather worried for a huge swelling on my dog's gums towards the rear of the mouth. He has a few molars extracted on each side of his dental arch because they were rotten to the core (he comes from a kennel). All seemed well until 2 weeks after the extraction I see the left side of his face with a swelling, and noticed a foul odor. I see this mass formed on the gums that is dark (same color as the gums), but not bleeding and is about marble size. It does not seem to hurt him.He is set to be operated Monday and the vet was not sure what is was but ruled out abscess. We ran all scans and bloodwork on him in September and was clear of any signs of cancer. I am worried that this is a malignant cyst. Is that possible? I mean it grew in only 2 weeks! Any experience on the matter is welcome. Thank youG'hound that is what I am hoping it is. Tommy, my Italian scent hound, is in good overall shape. No weight loss, no loss of appetite, lazy when indoors as always and willing to walk outside. He is 11 years old. No pus or blood from that growth and it is only on one side of the mouth. I am amazed at the rate of growth, and it is coming out Monday morning. Just hope that surgery doesn't not trigger just another mass.

It could be an Epulis which can often be benign. It needs to come off asap and the vet needs cytology reports. Physical trauma at injection sites can sometimes start abnormal cell growth that develops into a rapidly growing cyst.