Hello, Happy 4th of July!
Your question has me very curious as to what this area under your dog's tongue looks like--I wish I could see it myself!
I can see why you are worried about cancer. Certainly this is always on the list anytime we see a mass or "swelling". However, there are many other possibilities, and blood testing is a good first step, to make sure the dog is healthy otherwise and there is not an underlying problem leading to this.
Here's my list of some possibilities based on what you have described:
*An infection in the soft tissue under the tongue (like as abscess)
*Inflammation or infection associated with foreign material stuck in the tissue; for instance some dogs can get a tiny piece of wood or a grass awn stuck somewhere in their mouth as as example, and this can lead to chronic infection
*some type of trauma or puncture wound to the area with resulting swelling or hematoma (a pocket of blood) and bruising
* a benign growth such as a cyst
* a malignant tumor (lower on my list)
*Other more rare possibilities such as a bleeding disorder, low platelets, etc.
In general, when we find a mass or swelling like this, the best way to get an answer is to sedate the dog and do fine needle aspirates (FNA) of the area, and look at the sample on a slide to see if we can get an idea of what it might be. This can be an excellent screening tool and sometimes provides the diagnosis. We can also take a biopsy (a small sample of the tissue of the area) and send it off to a pathologist for an analysis. While the dog is sedated, we can thoroughly explore the area and see if we find anything else unusual (such as foreign material).
I would suggest sedation and further exam of this area, and possible aspirates and/or biopsy as the next step to getting an answer.
I hope this is helpful for you. Whippets are great little dogs. I don't see many of them but they are fun to work with. Thank you and happy holiday again!