pillowmutt,
It sounds like it is not an infection, as you have had a course of a very good antibiotic (azithromycin), which would usually have taken care of a localized infection.
The fact that it has very little pain is actually a very good sign, as most malignancies are painful.
Actually, what it sounds like is an epidermal inclusion cyst (EIC). These cysts are very common. What they are, is a cyst of the usual skin secretions. What happens is, that for some reason, a few epidermal cells get pushed down below the skin surface. But, they continue to make their usual oils and other secretions. So, the oils, etc have nowhere to go, so a cyst is formed, containing these secretions.
These cysts can vary from very hard to actually water balloon feeling, sort of firm but mushy. They can decrease in size if the body resorbs some of the oils, then can enlarge, if the secretions increase.
Usually, if the cyst is in a location that is continually bumped or pressed upon, or if it bothers the patient, it is usually surgically excised. These cysts are very common in the palm of the hand, around the scalp, and in the axilla (armpit).
If the cyst was not very large when the CT scan was done, it could have very easily not shown up on the scan. An MRI would probably have been a better imaging study, as it looks at soft tissues better than the CT scan. The CT scan actually uses x-rays, so it is not as good at looking at soft tissues as the MRI (which uses a super coiled magnet).
But, you may have to have an excisional biopsy to find out what this is exactly. It sounds like it is in a fairly accessible location, and in a place that the scar could be hidden in the folds around the nostrils.
Hope it turns out to be something benign and easily treated. Good luck.