What's this bump behind my ears?

For years now, I’ve had this bump behind my ears. It comes and goes depending on how stressed I am. When I’m stressed out, it swells up. When I don’t have anything to worry about, it goes away. What is it? Is it a lymph node? Aren’t those more related to your (physical) health? My sister also has the same thing, but it’s on her arm. What the heck is wrong with my family? Has anyone else heard of such a thing?