A swelling in the lower lip.
A mucocle is not to be confused with a cold sore from herpes simplex (Type 2) infection. The former is benign and painless while the latter implies that you've been naughty and have been putting things in your mouth that you should not.
A mucocele is a collection of saliva trapped within the soft tissue due to a ruptured minor salivary gland. If you run your tongue along the inner lining of your lips, you will feel small bumpy lumps that are meaty and nodular (that's the minor salivary gland). Do NOT under any circumstances bite or suck at it. Apparently, that's how I got this swelling (from chewing my lower lip) The end result is a round fluctuating nodule with a bluish tinge at the surface. You can't pop it like a pimple and it grows bigger if you keep worrying it.
If the darn thing grows bigger it is likely to be due to a blocked salivary duct and it's time to go for surgery. The procedure is to excise the salivary gland involved and send for biopsy (to rule out oral cancer). This costs RM400-450 done by an oral/dental surgeon. The minor operation takes less than 1/2 hour and recovery is expected within two days. However, a mucocele should be able to resolve spontaneously without intervention. Just leave it alone and it should disappear by its own little self after 3-6 weeks.... which was what happened to mine after some reassurance from the good doctor.
Anyways, surgery is an invasive procedure and a mucocele is likely to recur after the initial surgery so it's best to adopt the wait and see approach. I couldn't live with it at first and wanted it out immediately. I'm glad the doctor advised against it - see now, that's why physicians can't heal themselves (plus they make the worst patients). Being in the sick role messes with objectivity and judgement.