The good news is........ you can relax

No reason to go to the ER unless you are bleeding to death or having a heart attack!!!
This sounds like nothing more than a simple fluid filled sac, pustule. Very much like a pimple. It is common to get them behind the ear. Your hair produces what is called sebum (oil for you lay people

) it is common to get some oil build up on the skin behind the ear. I have had them numerous times!!!
If it were a tumor you would have all sorts of different symptoms. Also, tumors don't necessarily feel like fluid in a sac, they are more fiborous.
Yes, go to the doctor, but it is nothing to freak out over. Most likely it is just a really big pimple. Sebaceous cyst is what it is called. If it were to burst you would most likely get some whitish/yellowish pus out and then clear/pinkish fluid, then it will scab over and go away.
Chill..... it will all be okay.
It will go away, like a pimple. If it is large and painful enough the doc can lance it to help ease the pressure, but it really should be nothing to worry about.