Your ig has an infection.. does not look like a cyst as those are usually above the skin but could be from a burn or bacteria infection possibly even fungal.. a combo of topical ointment + antibiotics are needed so a vet visit is a must asap. Until you get go vet use ointment like neosporin. This alone wont work as that has likely hit the blood stream. Do yourself a favor and take a marker to the edge of that and mark it.. if it goes past the mark then it gives a good idea how far it advances. The marker mark will go away after shed.