I have had this lump on my head for years now and it has continuously gotten bigger. Its under my skin by my hair line directly behind my left ear. Its more than doubled in size, maybe the size of a nickle. recently it has started hurting but its not inflamed and deff not a pimple becuase its deep under my skin. Any ideas what this could be please?

I also have horrible allergies so i dont know if this could be in relation but i didnt think so.