What Does A Lump On Back Of Neck Indicate?

I noticed a lump on the back of my neck, on my left side, just above my hairline , it s a hard lump that s under the skin, id say it s kidney bean size! I ve never noticed it before and only recently felt it when rubbing my neck due to a headache, I ve suffered with migraines for about a year but this feels different , it s a headache but the main pressure feels like it s at the base of my head at the back ! Resting on anything (like a pillow) makes my headache worse . It s been like this for about 4 days now and haven t seen a doctor because I m not registered . Any ideas what It could be??