I had the same thing bro, on the side.
Its an inflamed lymph node, 99% of the time its not a problem at all and will likely reduce in size and firmness over time.
Less then 1% of the time it can be a sign of some sort of cancer or disease.
Are you alergic to anything.. a common cause of this is when someone who is alergic to cats gets exposed to one, your lymph nodes will react and become inflamed, often times it is just one side of the next not both. Any allergen can cause this same reaction, cat dander just seems to be one of the most common causes. Could have been something you ate, pollen, grass, etc.
Seeing a doc is a good idea, they will take note of the current size and want to see you again in a few weeks. If it doesn't change, no big deal, if it goes away, better deal, if it gets worse they will biopsy... a biopsy of a lymph node on the neck is involved, usually general anesthesia is used, so its a last resort.. and even if it is biopsied.. odds are you are fine anyway.