Thank you for posting your question here, I will answer it as best I can.
Did you hit your head anywhere recently? Or have any kind of injury
to your head?
If not, yes you should see a doctor as soon as possible.
Yes, it could be nothing serious but sudden painful lumps developing on the back of your head is something to get checked up. Better safe than sorry.
They may also run some tests on you like a blood test
and an MRI/CT to find out what is going on.
Again I could be wrong and its nothing to worry about, but why risk it? Go to a doctor and get it checked.
I hope this answered your question.