Any lump should be checked by a doctor. Most probably it is due to a swollen lymph node due to a boil or skin abscess. I had one that took two weeks to go away and got worse and worse for a week. It hurt to even raise my arm and then it gradually got better until it went away.
Hopefully it will pass away soon.
i've had those before.. as disgusting as it sounds, i pop them and they're full of a sweaty-smelling deodorant. i use solids so i guess the build-up of using it for so long and it getting into the pores when i sweat and they opened stated it. since yours is not exactly in the armpit, it is possible to be a swollen lymph node.
if you're not into risking popping it, as jdseouza said, hopefully it will pass soon. (:
This could be anything really, if it is causing discomfort see your doctor and get it checked out/removed.