Sounds like a cyst, some people call it a 'blind pimple'. It really is just a big pimple under the skin that never comes to a white/black head.
Warm compresses, topical or Rx antibiotic and benzoil peroxide should get rid of it in a few days. If it doesn't go away a dermatologist can take care of it for you.
DON'T try to pop it!
It'll rupture under the skin and cause a huge infection.
(if the cyst is[B] in[/B] your lip, even partially, you need to see a doctor right away)