Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry that you have been waiting for a response, but your requested expert isn't online which delayed your question coming up on the list for all to answer. I would like to help if you are still interested in an opinion.
I understand that you are concerned about a lump that seems to increase and decrease in size near Hallie's whiskers.
There are oil and scent glands near the whiskers, so it is possible that this is related to a blocked duct that sometimes reopens, thus the decrease in size. As it refills it increases in size again.
Another possible cause is a benign growth called a histiocytoma. These growths pop up quickly, grow quickly for a month or so and then tend to regress and resolve on their own within 2 to 3 months. They are most common in young dogs on the face, ears and legs and paws. But older dogs can get them too and if they scratch them they may not resolve.
If the dog rubs them or scratches they will hang around longer or can get infected. If she is rubbing or scratching at it I would recommend an E-collar for her to try and stop her from scratching any more. If the bump doesn't resolve completely in 3 months it should be surgically removed as it is unlikely to go away on its own at that point and can lead to a secondary infection. You don't say how long this has been an issue for her, so if this has been long term, more than 6 months a histiocytoma is less likely.
The most likely possibility is a tumor called a mast cell tumor. These tumors do change in size rapidly if they are bumped or irritated. They will increase and decrease in size, but never fully resolve. They have variable degrees of malignancy, some can be treated with just surgical removal, some are quite aggressive and will spread deep into tissue and to local lymph nodes. The only way to know for sure how malignant the tumor is is to remove it and have it biopsied. Unfortunately those on the face and groin do tend to behave in a more malignant manner.
Other tumors that are possible but much less likely, include a basal cell tumor (these are much less common in dogs then in people) or a melanoma. Melanomas tend to be fairly benign behaving in the skin in dogs and respond to surgical removal. A plasma cell tumor is another possibility. None of these will increase or decrease in size however.
Of all of these tumors none resolve completely on their own except a histiocytoma. Mast cell tumors will change in size if they are bumped, and then go down slightly in size but never fully resolve on their own.
I recommend that your veterinarian perform a fine needle aspirate on the bump. He or she puts a tiny needle in the bump, draws cells out, and looks at the cells under the microscope to identify them.
Please let me know if you have any further questions.