The fact it hurts and just popped up, is in your favour.
If it was there for many months and was not tender.....then it might be serious
Might be some infection or wart type thing, although warts usually don't hurt.
Gear can give absesses and make a lymph gland swell for a while.
I had the exact same thing you describe (behind the ear) and it went away after about 2 weeks.