probably just a mild infection,
mumps can be in your system for a month without showing symptoms, and the area it is is where you described, below and in front of the ears, but if you say its longer than that and you had the MMR vaccine it's unlikely to be mumps. By the way , testicles getting big is NOT a nessecary symptom like someone above said, it happens to about a third of men who get it.
Id say its just an infection, if its only on one side, may possibly be glandular fever, in which case it may spread to other glands like below your jawbone. If it begins to hurt when you eat or swallow, or you feel feverish like hot but shivering, then go see a doctor. But I dont think it should be urgent as both mumps and glandular fever are viral infections and the only thing a doctor could do I think is to give you prescribed painkillers, although maybe antibiotics when you recover. if you get other syptoms like headache or nausea or stuff like that then go to a doctor because it might be more serious or get complications (which can be serious)
at the moment if its just a lump that doesn't hurt they can't tell much, I went with a lump under my chin to the left and they said its not mumps probably but can't tell much what it is. They might do a blodd test which is quite fast to come back, but recently I think theyre not doing them because of the mumps outbreak, just doing swab tests (in the mouth) to gather some data on where it is spreading.
if you want to, go to a doctor, but my guess is they cant do anything exept say come back when it hurts or get worse.