I just had a similar knot develop behind my ear... It was very sore and appeared out of no where. So I "googled" it and found it was most likely a lymph node that was enlarged due to some sort of infection my body was fighting... I've had some allergies recently.
It has already decreased by 80% in size in a couple of days... The info I found said it would remain there for some time until all of the antibodies/while blood cells dissipated. Or it may stay enlarged.
If you have had a cold, virus, allergies, tooth ailments... Such things can cause your lymph nodes to kick into gear.
If its been there a long time, see your doctor. You don't want to mess around with something that could be serious and possibly delay treatment.
Best regards