This could be anything really, if it is causing discomfort see your doctor and get it checked out/removed.
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. (:
Hi,
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.