Since when is the swelling? Is there fever, weight loss, and loss of appetite? Is the swelling mobile or fixed? How does it feels; soft, firm, hard or fluid filled? Is the skin adherent to the swelling?
Cellulitis can have significant pain and can make the tissue feel tight and firm. CT scan is a very good investigation to distinguish the tissues involved. A sudden appearance of the swelling always denote an infective pathology. Read the resource;
Antibiotics take 3-4 days to show the response and benefit.
Please feel free for your follow up questions.