Hello and hope you are doing well.
Its unlikely that a gas pocket will appear as a swelling. Its may not be from the colon. Do you have pain on touching the swelling? If so then it could be a skin infection or an abscess. It could also be a lipoma. But to exactly pinpoint the pathology you will need to consult your primary care physician who will examine you and may ask for blood tests. If the need arises he may refer you to a dermatologist.
Hope this helped and do keep us posted.