Hi there ~
I understand your concerns. The lump on the back of your neck seems concerning and seems like it is the reason for your horrible headaches
are common and it might be due to a blood disorder. I also think you are doing the right thing by seeing a doctor to have your B12 level corrected. I also hope that you see a neurologist
to find out what the reason is.
I hope this helps. Take care and have a lovely day!